diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt
index a12d49cb..5800a380 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt
@@ -140,9 +140,11 @@ object SafDownloadHandler {
                 val response = backend.downloadByStrategy(req.toString())
                 val respObj = JSONObject(response)
                 if (respObj.optBoolean("success", false)) {
+                    val actualExt = normalizeExt(respObj.optString("actual_extension", ""))
+                        .ifBlank { outputExt }
                     val resolvedFileName = respObj.optString("resolved_file_name", "")
                         .trim()
-                        .let { if (it.isNotEmpty()) forceFilenameExt(it, outputExt) else fileName }
+                        .let { forceFilenameExt(it.ifBlank { fileName }, actualExt) }
                     val reportedPath = respObj.optString("file_path", "").trim()
                     if (reportedPath.isEmpty() || reportedPath.startsWith("/proc/self/fd/")) {
                         respObj.put("file_path", workingFile.absolutePath)
@@ -159,8 +161,8 @@ object SafDownloadHandler {
                     )
                     respObj.put("saf_relative_dir", resolvedDir)
                     respObj.put("saf_tree_uri", treeUriStr)
-                    respObj.put("saf_output_ext", outputExt)
-                    respObj.put("saf_final_mime_type", mimeType)
+                    respObj.put("saf_output_ext", actualExt)
+                    respObj.put("saf_final_mime_type", mimeTypeForExt(actualExt))
                 } else {
                     workingFile.delete()
                 }
diff --git a/integration_test/download_output_container_test.dart b/integration_test/download_output_container_test.dart
new file mode 100644
index 00000000..1fd9ce0d
--- /dev/null
+++ b/integration_test/download_output_container_test.dart
@@ -0,0 +1,192 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:ffmpeg_kit_flutter_new_audio/ffmpeg_kit.dart';
+import 'package:ffmpeg_kit_flutter_new_audio/return_code.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:spotiflac_android/models/download_result.dart';
+import 'package:spotiflac_android/models/extension.dart';
+import 'package:spotiflac_android/models/settings.dart';
+import 'package:spotiflac_android/models/track.dart';
+import 'package:spotiflac_android/services/download_metadata_embedding.dart';
+import 'package:spotiflac_android/services/download_request_payload.dart';
+import 'package:spotiflac_android/services/ffmpeg_service.dart';
+import 'package:spotiflac_android/services/platform_bridge.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+  const channel = MethodChannel('com.zarz.spotiflac/backend');
+  late Directory root;
+  late Directory output;
+  late Directory sources;
+
+  setUp(() async {
+    root = await Directory.systemTemp.createTemp('download-container-');
+    output = await Directory('${root.path}/output').create();
+    sources = await Directory('${root.path}/extensions').create();
+    final data = await Directory('${root.path}/data').create();
+    await PlatformBridge.initExtensionSystem(
+      sources.path,
+      data.path,
+      masterKey: base64Encode(List<int>.filled(32, 1)),
+      lyricsProviders: const [],
+      lyricsFetchOptions: const {},
+      allowedDirectories: [output.path],
+    );
+    final implementations = await PlatformBridge.getBackendImplementations();
+    expect(implementations['downloads'], 'rust');
+    expect(implementations['extensions'], 'rust');
+  });
+
+  tearDown(() async {
+    await PlatformBridge.cleanupExtensions();
+    await root.delete(recursive: true);
+  });
+
+  for (final (format, returnedExtension) in [
+    ('m4a', 'm4a'),
+    ('m4a', 'mp4'),
+    ('m4a', 'flac'),
+    ('mp3', 'mp3'),
+    ('flac', 'flac'),
+  ]) {
+    for (final explicitWorkingPath in [false, true]) {
+      testWidgets(
+        'foreground $format returned as $returnedExtension (working: $explicitWorkingPath)',
+        (tester) async {
+          final fixture = File('${root.path}/fixture.$format');
+          final codec = switch (format) {
+            'm4a' => 'aac',
+            'mp3' => 'libmp3lame',
+            _ => 'flac',
+          };
+          final session = await FFmpegKit.executeWithArguments([
+            '-y',
+            '-f',
+            'lavfi',
+            '-i',
+            'sine=frequency=440:sample_rate=44100',
+            '-t',
+            '0.25',
+            '-c:a',
+            codec,
+            fixture.path,
+          ]);
+          expect(
+            ReturnCode.isSuccess(await session.getReturnCode()),
+            true,
+            reason: await session.getOutput(),
+          );
+          final audio = await fixture.readAsBytes();
+          final provider = await Directory(
+            '${sources.path}/example.audio',
+          ).create();
+          await File('${provider.path}/manifest.json').writeAsString(
+            jsonEncode({
+              'name': 'example.audio',
+              'version': '1',
+              'description': 'Generic native-container fixture',
+              'type': ['download_provider'],
+              'permissions': {'file': true},
+              'skipMetadataEnrichment': true,
+            }),
+          );
+          await File('${provider.path}/index.js').writeAsString('''
+registerExtension({
+  checkAvailability() { return { available: true, track_id: 'track' }; },
+  download(trackId, quality, outputPath) {
+    const path = outputPath.slice(0, outputPath.lastIndexOf('.')) + '.$returnedExtension';
+    file.writeBytes(path, ${jsonEncode(base64Encode(audio))});
+    return { success: true, file_path: path };
+  }
+});
+''');
+          await PlatformBridge.loadExtensionsFromDir(sources.path);
+          await PlatformBridge.setExtensionEnabled('example.audio', true);
+          const settings = AppSettings(
+            nativeDownloadWorkerEnabled: false,
+            embedMetadata: true,
+            embedLyrics: false,
+            embedReplayGain: false,
+          );
+          final request = DownloadRequestPayload(
+            service: 'example.audio',
+            providerTrackId: 'track',
+            trackName: 'Track',
+            artistName: 'Artist & Guest',
+            albumName: 'Album',
+            outputDir: output.path,
+            filenameFormat: '{title}',
+            outputExt: '.flac',
+            // MP4 bytes named .flac must never enter the Rust FLAC tag writer.
+            embedMetadata: format == 'm4a' && returnedExtension == 'flac',
+            embedLyrics: false,
+            useExtensions: true,
+            useFallback: false,
+          );
+          // These are the foreground queue's real bridge and explicit local
+          // working-path contract. No service-owned worker or channel mock.
+          final Map<String, dynamic> response;
+          if (explicitWorkingPath) {
+            final raw = await channel.invokeMethod<String>(
+              'downloadByStrategy',
+              jsonEncode({
+                ...request.toJson(),
+                'output_path': '${output.path}/working.flac',
+              }),
+            );
+            response = Map<String, dynamic>.from(jsonDecode(raw!) as Map);
+          } else {
+            response = await PlatformBridge.downloadByStrategy(
+              payload: request,
+            );
+          }
+          expect(response['success'], true, reason: response.toString());
+          final result = DownloadResult.fromMap(response);
+          final suffix = '.$format';
+          final stem = explicitWorkingPath ? 'working' : 'Track';
+          expect(result.filePath, '${output.path}/$stem$suffix');
+          expect(result.outputExtension(), suffix);
+          expect(response['resolved_file_name'], 'Track$suffix');
+          final downloaded = File(result.filePath!);
+          expect(await downloaded.readAsBytes(), audio);
+          if (format != 'flac') {
+            expect(await File('${output.path}/$stem.flac').exists(), false);
+          }
+          final audioCodec = format == 'mp3' ? 'mp3' : codec;
+          expect(
+            await FFmpegService.probePrimaryAudioCodec(downloaded.path),
+            audioCodec,
+          );
+          // The same tag writer used by the foreground queue must preserve
+          // the native stream while embedding source credits into its container.
+          await DownloadMetadataEmbedding(onReplayGain: (_, _, _) {}).embed(
+            downloaded.path,
+            const Track(
+              id: 'example:track',
+              name: 'Track',
+              artistName: 'Artist & Guest',
+              albumName: 'Album',
+              duration: 1,
+            ),
+            settings: settings,
+            extensionState: const ExtensionState(),
+            format: format,
+          );
+          final metadata = await PlatformBridge.readFileMetadata(
+            downloaded.path,
+          );
+          expect(metadata['title'], 'Track');
+          expect(metadata['artist'], 'Artist & Guest');
+          expect(metadata['album'], 'Album');
+          expect(
+            await FFmpegService.probePrimaryAudioCodec(downloaded.path),
+            audioCodec,
+          );
+        },
+      );
+    }
+  }
+}
diff --git a/rust_backend/crates/extensions/src/backend/download_container_tests.rs b/rust_backend/crates/extensions/src/backend/download_container_tests.rs
new file mode 100644
index 00000000..947efd1f
--- /dev/null
+++ b/rust_backend/crates/extensions/src/backend/download_container_tests.rs
@@ -0,0 +1,142 @@
+use super::*;
+use crate::RuntimeLimits;
+use base64::Engine;
+
+#[test]
+fn provider_format_fields_and_returned_path_override_the_planned_extension() {
+    let request = DownloadRequest {
+        track_name: "Track".into(),
+        filename_format: "{title}".into(),
+        output_ext: ".flac".into(),
+        ..Default::default()
+    };
+    let manifest = ExtensionManifest::default();
+    for (result, expected) in [
+        (json!({"file_path":"staged/Track.m4a"}), ".m4a"),
+        (json!({"file_path":"staged/Track.MP3"}), ".mp3"),
+        (json!({"file_path":"staged/Track.opus"}), ".opus"),
+        (json!({"file_path":"staged/Track.mp4"}), ".m4a"),
+        (json!({"file_path":"staged/Track.flac"}), ".flac"),
+        (
+            json!({"file_path":"staged/Track.mp3","actual_container":"audio/mpeg"}),
+            ".mp3",
+        ),
+        (
+            json!({"file_path":"staged/Track.m4a","actual_extension":"audio/mp4"}),
+            ".m4a",
+        ),
+        (
+            json!({"file_path":"staged/Track.flac","actual_container":"m4a"}),
+            ".m4a",
+        ),
+        (
+            json!({"file_path":"staged/Track.flac","output_extension":" MP4 "}),
+            ".m4a",
+        ),
+        (
+            json!({"file_path":"staged/Track.m4a","actual_extension":".mp3","output_extension":".flac"}),
+            ".mp3",
+        ),
+        (json!({}), ".flac"),
+    ] {
+        let response = success(
+            &request,
+            &result,
+            "planned/Track.flac",
+            false,
+            &manifest,
+            &|| Ok(()),
+        )
+        .unwrap();
+        assert_eq!(response["actual_extension"], expected);
+        assert_eq!(response["resolved_file_name"], format!("Track{expected}"));
+    }
+}
+
+#[test]
+fn downloads_publish_the_actual_container_for_regular_and_explicit_working_paths() {
+    // Header-only payloads verify byte-preserving publication; real decodable
+    // audio and Flutter metadata finalization are exercised on devices.
+    for (returned_extension, payload, expected) in [
+        ("m4a", b"\0\0\0\x18ftypM4A fixture".as_slice(), ".m4a"),
+        ("mp4", b"\0\0\0\x18ftypM4A fixture".as_slice(), ".m4a"),
+        ("mp3", b"ID3example audio".as_slice(), ".mp3"),
+        ("opus", b"OggSexample audio".as_slice(), ".opus"),
+        // A legacy provider may leave the requested suffix on MP4 bytes.
+        ("flac", b"\0\0\0\x18ftypM4A fixture".as_slice(), ".m4a"),
+    ] {
+        for explicit_path in [false, true] {
+            let directory = tempfile::tempdir().unwrap();
+            let source = directory.path().join("sources/example.audio");
+            std::fs::create_dir_all(&source).unwrap();
+            std::fs::write(
+                source.join("manifest.json"),
+                json!({"name":"example.audio","version":"1",
+                    "description":"Generic native-container fixture","type":["download_provider"],
+                    "permissions":{"file":true},"skipMetadataEnrichment":true})
+                .to_string(),
+            )
+            .unwrap();
+            let encoded = base64::engine::general_purpose::STANDARD.encode(payload);
+            std::fs::write(
+                source.join("index.js"),
+                format!(
+                    r#"registerExtension({{checkAvailability(){{return {{available:true,track_id:'track'}};}},
+                    download(trackId,quality,outputPath){{
+                    const path=outputPath.replace(/\.[^.]+$/,'.{returned_extension}');
+                    file.writeBytes(path,{encoded});
+                    return {{success:true,file_path:path}};}}}});"#,
+                    encoded = json!(encoded),
+                ),
+            )
+            .unwrap();
+            let backend = Backend::new(
+                &directory.path().join("sources"),
+                &directory.path().join("data"),
+                "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=",
+                "1",
+                RuntimeLimits::default(),
+            )
+            .unwrap();
+            backend.load_all().unwrap();
+            backend.set_enabled("example.audio", true).unwrap();
+            backend.download_manifest("example.audio").unwrap();
+            let output = directory.path().join("output");
+            std::fs::create_dir_all(&output).unwrap();
+            backend
+                .environment()
+                .set_allowed_download_directories(std::slice::from_ref(&output))
+                .unwrap();
+            let mut request = json!({"service":"example.audio","provider_track_id":"track",
+                "use_extensions":true,"use_fallback":false,"output_dir":output,
+                "track_name":"Track","artist_name":"Artist","album_name":"Album",
+                "filename_format":"{title}","output_ext":".flac","quality":"LOSSLESS",
+                "embed_metadata":returned_extension == "flac","embed_lyrics":false});
+            if explicit_path {
+                request["output_path"] = output.join("working.flac").to_string_lossy().into();
+            }
+            let raw = backend
+                .download_by_strategy(&request.to_string(), &|| Ok(()))
+                .unwrap();
+            let response: Value = serde_json::from_str(&raw).unwrap();
+            assert_eq!(response["success"], true, "{response}");
+            assert_eq!(response["actual_extension"], expected, "{response}");
+            assert_eq!(response["resolved_file_name"], format!("Track{expected}"));
+            let stem = if explicit_path { "working" } else { "Track" };
+            let published = output.join(format!("{stem}{expected}"));
+            assert_eq!(response["file_path"], published.to_string_lossy().as_ref());
+            assert_eq!(std::fs::read(&published).unwrap(), payload);
+            assert!(!output.join(format!("{stem}.flac")).exists());
+            // Resolving the corrected suffix must never replace a sibling.
+            let second: Value = serde_json::from_str(
+                &backend
+                    .download_by_strategy(&request.to_string(), &|| Ok(()))
+                    .unwrap(),
+            )
+            .unwrap();
+            assert_eq!(second["success"], false, "{second}");
+            assert_eq!(std::fs::read(&published).unwrap(), payload);
+            backend.shutdown();
+        }
+    }
+}
diff --git a/rust_backend/crates/extensions/src/backend/downloads.rs b/rust_backend/crates/extensions/src/backend/downloads.rs
index 153e0aae..68ebe0dd 100644
--- a/rust_backend/crates/extensions/src/backend/downloads.rs
+++ b/rust_backend/crates/extensions/src/backend/downloads.rs
@@ -842,10 +842,15 @@ impl Backend {
         let mut response = success(request, &result, &path, false, manifest, &check)?;
         let mut resolved = request.clone();
         resolved.output_ext = text(&response, "actual_extension").to_owned();
-        let mut final_path = output_path(&resolved, &directory, text(&response, "album"), &check)?;
         if !request.output_path.is_empty() {
-            final_path = request.output_path.clone();
+            // SAF uses a local working path too. Keep its directory and stem,
+            // but use the provider's actual container before final publication.
+            resolved.output_path = Path::new(&request.output_path)
+                .with_extension(resolved.output_ext.trim_start_matches('.'))
+                .to_string_lossy()
+                .into_owned();
         }
+        let final_path = output_path(&resolved, &directory, text(&response, "album"), &check)?;
         // Keep the original lock for an unchanged destination. Release it
         // before acquiring a different path lock to avoid lock-order cycles.
         let destination = files.resolve_legacy(&final_path)?;
@@ -874,7 +879,7 @@ impl Backend {
         // ReplayGain, decryption/conversion, external LRC and extension hooks
         // are executed by the existing Android/Dart finalizers. This is the
         // Go backend's local FLAC embed step; provider lyrics are reused.
-        let embed = request.embed_metadata && returned.to_ascii_lowercase().ends_with(".flac");
+        let embed = request.embed_metadata && resolved.output_ext == ".flac";
         let cover = if embed {
             let url = [text(&response, "cover_url"), &request.cover_url]
                 .into_iter()
@@ -1488,8 +1493,18 @@ fn probe_result(
         let mut header = [0; 8];
         // Quality is probed only for FLAC/MP4. Missing album folders may use
         // tags from any supported container, including undecoded audio.
-        let quality = file.read_exact(&mut header).is_ok()
-            && (&header[..4] == b"fLaC" || &header[4..] == b"ftyp");
+        let has_header = file.read_exact(&mut header).is_ok();
+        let container = if has_header && &header[..4] == b"fLaC" {
+            "flac"
+        } else if has_header && &header[4..] == b"ftyp" {
+            "m4a"
+        } else {
+            ""
+        };
+        let quality = !container.is_empty();
+        if quality {
+            result["actual_container"] = container.into();
+        }
         let album = album_pending && text(result, "album").trim().is_empty();
         if (quality || album)
             && file.seek(SeekFrom::Start(0)).is_ok()
@@ -1699,14 +1714,37 @@ fn success(
             response[key] = value.clone();
         }
     }
+    let container = text(result, "actual_container")
+        .trim()
+        .trim_start_matches('.');
+    let container = if [
+        "flac", "m4a", "mp4", "mp3", "opus", "ogg", "aac", "wav", "aiff", "aif", "aifc",
+    ]
+    .iter()
+    .any(|extension| container.eq_ignore_ascii_case(extension))
+    {
+        container
+    } else {
+        ""
+    };
     let extension = [
         text(result, "actual_extension"),
         text(result, "output_extension"),
+        container,
     ]
     .into_iter()
     .map(|value| value.trim().to_lowercase())
-    .find(|value| !value.is_empty())
-    .unwrap_or_default();
+    .find(|value| !value.is_empty() && !value.contains(['/', '\\']))
+    .unwrap_or_else(|| {
+        // Providers may only return a renamed file_path, without explicit
+        // format fields. The planned path still carries the requested quality.
+        [text(result, "file_path"), path]
+            .into_iter()
+            .filter_map(|path| Path::new(path).extension())
+            .map(|extension| extension.to_string_lossy().trim().to_lowercase())
+            .find(|extension| !extension.is_empty())
+            .unwrap_or_default()
+    });
     if !extension.is_empty() {
         let extension = format!(".{}", extension.trim_start_matches('.'));
         response["actual_extension"] = if extension == ".mp4" {
@@ -1719,16 +1757,8 @@ fn success(
     if !text(result, "isrc").trim().is_empty() {
         named.isrc = text(result, "isrc").trim().into();
     }
-    let extension = if text(&response, "actual_extension").is_empty() {
-        Path::new(path)
-            .extension()
-            .unwrap_or_default()
-            .to_string_lossy()
-            .into_owned()
-    } else {
-        text(&response, "actual_extension").into()
-    };
-    response["resolved_file_name"] = filename(&named, &extension, check)?.into();
+    response["resolved_file_name"] =
+        filename(&named, text(&response, "actual_extension"), check)?.into();
     response["resolved_album_folder"] = album_folder(request, text(&response, "album")).into();
     Ok(response)
 }
@@ -1792,3 +1822,7 @@ mod tests;
 #[cfg(test)]
 #[path = "download_account_tests.rs"]
 mod account_tests;
+
+#[cfg(test)]
+#[path = "download_container_tests.rs"]
+mod container_tests;