diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
index fbb49e1f..563af219 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
@@ -462,7 +462,7 @@ internal object NativeFinalizationPolicy {
         return trimmed
     }
 
-    private fun normalizeExtension(extension: String?): String {
+    internal fun normalizeExtension(extension: String?): String {
         val trimmed = extension?.trim().orEmpty()
         if (trimmed.isEmpty()) return ""
         val lower = trimmed.lowercase(Locale.ROOT)
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 8ee98f1c..e2fd3b3a 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt
@@ -632,11 +632,9 @@ object SafDownloadHandler {
         }
     }
 
-    internal fun normalizeExt(ext: String?): String {
-        val trimmed = ext?.trim().orEmpty()
-        if (trimmed.isEmpty()) return ""
-        return if (trimmed.startsWith(".")) trimmed.lowercase(Locale.ROOT) else ".${trimmed.lowercase(Locale.ROOT)}"
-    }
+    // Same result for every input; pinned by ExtensionMimeTablesTest.
+    internal fun normalizeExt(ext: String?): String =
+        NativeFinalizationPolicy.normalizeExtension(ext)
 
     internal fun mimeTypeForExt(ext: String?): String {
         return when (normalizeExt(ext)) {
diff --git a/android/app/src/test/kotlin/com/zarz/spotiflac/ExtensionMimeTablesTest.kt b/android/app/src/test/kotlin/com/zarz/spotiflac/ExtensionMimeTablesTest.kt
new file mode 100644
index 00000000..abcf6543
--- /dev/null
+++ b/android/app/src/test/kotlin/com/zarz/spotiflac/ExtensionMimeTablesTest.kt
@@ -0,0 +1,145 @@
+package com.zarz.spotiflac
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+// Pins the extension and MIME tables used by the SAF and finalizer paths.
+// They are not inverse maps: "audio/ogg" names ".opus", while ".ogg" and
+// ".opus" both map back to "audio/ogg", and ".aac" has no MIME entry.
+class ExtensionMimeTablesTest {
+    @Test
+    fun extFromFileNameMatchesLowercaseSuffixesOnly() {
+        for ((input, expected) in listOf(
+            "song.m4a" to ".m4a",
+            "song.mp4" to ".mp4",
+            "song.aac" to ".aac",
+            "song.mp3" to ".mp3",
+            "song.opus" to ".opus",
+            "song.flac" to ".flac",
+            "song.ogg" to ".ogg",
+            "song.wave" to ".wav",
+            "song.wav" to ".wav",
+            "song.aiff" to ".aiff",
+            "song.aifc" to ".aifc",
+            "song.aif" to ".aif",
+            "song.dsf" to ".dsf",
+            "song.dff" to ".dff",
+            ".flac" to ".flac",
+            "my.mp3.flac" to ".flac",
+            "song.wv" to "",
+            "song.cue" to "",
+            "song.lrc" to "",
+            "song.flac.tmp" to "",
+            "song.flac " to "",
+            "SONG.FLAC" to "",
+            "song.Mp3" to "",
+            "flac" to "",
+            "" to "",
+            " " to "",
+        )) {
+            assertEquals(input, expected, extFromFileName(input))
+        }
+    }
+
+    @Test
+    fun extFromMimeTypeMatchesExactTypesOnly() {
+        for ((input, expected) in listOf(
+            "audio/mp4" to ".m4a",
+            "audio/aac" to ".aac",
+            "audio/eac3" to ".m4a",
+            "audio/ac3" to ".m4a",
+            "audio/ac4" to ".m4a",
+            "audio/mpeg" to ".mp3",
+            "audio/ogg" to ".opus",
+            "audio/flac" to ".flac",
+            "audio/wav" to ".wav",
+            "audio/x-wav" to ".wav",
+            "audio/wave" to ".wav",
+            "audio/vnd.wave" to ".wav",
+            "audio/aiff" to ".aiff",
+            "audio/x-aiff" to ".aiff",
+            null to "",
+            "" to "",
+            " " to "",
+            "AUDIO/FLAC" to "",
+            " audio/flac" to "",
+            "audio/x-flac" to "",
+            "audio/opus" to "",
+            "audio/mp3" to "",
+            "application/octet-stream" to "",
+        )) {
+            assertEquals(input.toString(), expected, extFromMimeType(input))
+        }
+    }
+
+    @Test
+    fun normalizersTrimLowercaseAndAddOneLeadingDot() {
+        for ((input, expected) in normalizerCases) {
+            assertEquals(input.toString(), expected, SafDownloadHandler.normalizeExt(input))
+            assertEquals(input.toString(), expected, NativeFinalizationPolicy.normalizeExtension(input))
+        }
+    }
+
+    @Test
+    fun mimeTypeForExtNormalizesBeforeLookup() {
+        val octetStream = "application/octet-stream"
+        for ((input, expected) in listOf(
+            ".m4a" to "audio/mp4",
+            "m4a" to "audio/mp4",
+            "M4A" to "audio/mp4",
+            ".mp4" to "audio/mp4",
+            ".mp3" to "audio/mpeg",
+            ".opus" to "audio/ogg",
+            ".ogg" to "audio/ogg",
+            ".flac" to "audio/flac",
+            " FLAC " to "audio/flac",
+            ".wav" to "audio/wav",
+            ".aiff" to "audio/aiff",
+            ".aif" to "audio/aiff",
+            ".aifc" to "audio/aiff",
+            ".lrc" to octetStream,
+            ".aac" to octetStream,
+            ".wave" to octetStream,
+            ".dsf" to octetStream,
+            ".dff" to octetStream,
+            ".wv" to octetStream,
+            "..flac" to octetStream,
+            "audio/flac" to octetStream,
+            null to octetStream,
+            "" to octetStream,
+            "  " to octetStream,
+        )) {
+            assertEquals(input.toString(), expected, SafDownloadHandler.mimeTypeForExt(input))
+        }
+    }
+
+    private val normalizerCases = listOf(
+        null to "",
+        "" to "",
+        "   " to "",
+        "\t\n" to "",
+        "m4a" to ".m4a",
+        "mp4" to ".mp4",
+        "aac" to ".aac",
+        "mp3" to ".mp3",
+        "opus" to ".opus",
+        "ogg" to ".ogg",
+        "flac" to ".flac",
+        "wav" to ".wav",
+        "aiff" to ".aiff",
+        "aif" to ".aif",
+        "aifc" to ".aifc",
+        "lrc" to ".lrc",
+        ".flac" to ".flac",
+        "FLAC" to ".flac",
+        ".FLAC" to ".flac",
+        " .Mp3 " to ".mp3",
+        "\tFlac\n" to ".flac",
+        "  m4a" to ".m4a",
+        "." to ".",
+        ".." to "..",
+        "..flac" to "..flac",
+        "tar.gz" to ".tar.gz",
+        "ÉXT" to ".éxt",
+    )
+}