diff --git a/android/app/src/androidTest/kotlin/com/zarz/spotiflac/NativeDownloadContainerTest.kt b/android/app/src/androidTest/kotlin/com/zarz/spotiflac/NativeDownloadContainerTest.kt
new file mode 100644
index 00000000..c8332c59
--- /dev/null
+++ b/android/app/src/androidTest/kotlin/com/zarz/spotiflac/NativeDownloadContainerTest.kt
@@ -0,0 +1,58 @@
+package com.zarz.spotiflac
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import androidx.test.platform.app.InstrumentationRegistry
+import java.io.File
+import org.json.JSONObject
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class NativeDownloadContainerTest {
+    @Test
+    fun misnamedMp4IsTaggedAndPublishedAsM4aWithoutOverwritingExistingAudio() {
+        val context = InstrumentationRegistry.getInstrumentation().targetContext
+        val root = File(context.cacheDir, "container-test-${System.nanoTime()}").apply { mkdirs() }
+        try {
+            val input = File(root, "track.flac")
+            val existing = File(root, "track.m4a").apply { writeText("existing download") }
+            val fixture = NativeDownloadFinalizer.runFFmpegArguments(arrayOf(
+                "-v", "error", "-f", "lavfi", "-i", "sine=frequency=997:sample_rate=44100",
+                "-t", "0.2", "-c:a", "aac", "-f", "mp4", input.path,
+            ))
+            assertTrue(fixture.second, fixture.first)
+            assertTrue(isMP4ContainerFile(input.path))
+            val request = JSONObject()
+                .put("contract_version", 1).put("item_id", "example-track")
+                .put("service", "example-provider").put("track_name", "Container test")
+                .put("artist_name", "Example artist").put("album_name", "Example album")
+                .put("quality", "LOSSLESS").put("storage_mode", "app")
+                .put("output_ext", ".flac").put("embed_metadata", true)
+            val result = NativeDownloadFinalizer.finalize(
+                context, "example-track", request.toString(), "{}",
+                JSONObject().put("success", true).put("file_path", input.path)
+                    .put("file_name", input.name),
+                "{\"save_download_history\":false}",
+            )
+            assertTrue(result.toString(), result.getBoolean("success"))
+            assertTrue(result.getBoolean("native_finalized"))
+            val output = File(result.getString("file_path"))
+            assertEquals("m4a", output.extension)
+            assertTrue(output.length() > 0)
+            assertTrue(isMP4ContainerFile(output.path))
+            assertFalse(input.exists())
+            assertEquals("existing download", existing.readText())
+            val probe = NativeDownloadFinalizer.runFFmpegArguments(arrayOf(
+                "-hide_banner", "-i", output.path, "-map", "0:a:0", "-f", "null", "-",
+            ))
+            assertTrue(probe.second, probe.first)
+            assertTrue(probe.second, probe.second.contains("Container test"))
+            assertTrue(probe.second, probe.second.contains("Audio: aac"))
+        } finally {
+            root.deleteRecursively()
+        }
+    }
+}
diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
index 6d8e09a0..a10845c3 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
@@ -760,6 +760,7 @@ object NativeDownloadFinalizer {
         if (!forceContainerConversion && requestedDecryptionExt.isNotBlank() && requestedDecryptionExt != ".flac") return
         val mayNeedContainerConversion = forceContainerConversion ||
             looksLikeM4a(state.filePath, state.fileName) ||
+            isMP4ContainerFile(state.filePath) ||
             state.filePath.startsWith("content://")
         if (!mayNeedContainerConversion) return
 
@@ -768,6 +769,7 @@ object NativeDownloadFinalizer {
         val output = buildOutputPath(localInput, ".flac")
         val stagedOutput = stagedConversionPath(output)
         var adoptedOutput = false
+        var createdOutput = false
         try {
             val codec = probePrimaryAudioCodec(localInput, shouldCancel)
             val isAlreadyNativeFlac = codec == "flac" && isNativeFlacFile(localInput)
@@ -794,6 +796,7 @@ object NativeDownloadFinalizer {
                     if (!promoteStagedConversion(stagedOutput, output)) {
                         throw IllegalStateException("failed to publish native FLAC output")
                     }
+                    createdOutput = true
                     output
                 }
                 embedBasicMetadata(context, nativeFlacOutput, input, "flac")
@@ -811,6 +814,7 @@ object NativeDownloadFinalizer {
             if (!promoteStagedConversion(stagedOutput, output)) {
                 throw IllegalStateException("failed to publish container conversion output")
             }
+            createdOutput = true
             // Keep metadata failures before adoption so the source survives
             // and the unsuccessful output is removed by the local cleanup.
             embedBasicMetadata(context, output, input, "flac")
@@ -819,7 +823,7 @@ object NativeDownloadFinalizer {
         } finally {
             if (!adoptedOutput) {
                 File(stagedOutput).delete()
-                File(output).delete()
+                if (createdOutput && output != localInput) File(output).delete()
             }
             if (deleteLocalInput) File(localInput).delete()
         }
@@ -846,14 +850,7 @@ object NativeDownloadFinalizer {
             codec == "opus" -> ".opus"
             else -> return
         }
-        val renamed = File(
-            currentFile.parentFile,
-            currentFile.name.dropLast(".flac".length) + newExt,
-        )
-        if (renamed.exists() && !renamed.delete()) {
-            Log.w(TAG, "Cannot adopt container extension; ${renamed.name} already exists")
-            return
-        }
+        val renamed = File(uniqueAutoConversionOutputPath(currentFile.path, newExt))
         if (!currentFile.renameTo(renamed)) {
             Log.w(TAG, "Failed to rename preserved container to ${renamed.name}")
             return
diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizerFFmpeg.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizerFFmpeg.kt
index d686065b..24077a0c 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizerFFmpeg.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizerFFmpeg.kt
@@ -35,7 +35,7 @@ import kotlin.math.pow
 
 // FFmpeg execution, probing, and container helpers for NativeDownloadFinalizer.
 
-internal fun NativeDownloadFinalizer.isMP4ContainerFile(path: String): Boolean {
+internal fun isMP4ContainerFile(path: String): Boolean {
     return try {
         File(path).inputStream().use { stream ->
             val header = ByteArray(12)