diff --git a/libraries/test_utils/src/main/java/androidx/media3/test/utils/FileUtil.java b/libraries/test_utils/src/main/java/androidx/media3/test/utils/FileUtil.java
index fda6bf4f5f..e716829870 100644
--- a/libraries/test_utils/src/main/java/androidx/media3/test/utils/FileUtil.java
+++ b/libraries/test_utils/src/main/java/androidx/media3/test/utils/FileUtil.java
@@ -16,6 +16,7 @@
 
 package androidx.media3.test.utils;
 
+import static androidx.media3.common.util.Assertions.checkState;
 import static com.google.common.truth.Truth.assertThat;
 
 import android.content.Context;
@@ -43,7 +44,7 @@ public final class FileUtil {
    */
   public static void assertFileHasColorTransfer(
       Context context, @Nullable String filePath, @C.ColorTransfer int expectedColorTransfer) {
-    Format videoTrackFormat = retrieveVideoTrackFormat(context, filePath);
+    Format videoTrackFormat = retrieveTrackFormat(context, filePath, C.TRACK_TYPE_VIDEO);
     @Nullable ColorInfo colorInfo = videoTrackFormat.colorInfo;
     @C.ColorTransfer
     int actualColorTransfer =
@@ -53,7 +54,9 @@ public final class FileUtil {
     assertThat(actualColorTransfer).isEqualTo(expectedColorTransfer);
   }
 
-  private static Format retrieveVideoTrackFormat(Context context, @Nullable String filePath) {
+  /** Returns {@linkplain Format track format} from the media file. */
+  public static Format retrieveTrackFormat(
+      Context context, @Nullable String filePath, @C.TrackType int trackType) {
     TrackGroupArray trackGroupArray;
     try {
       trackGroupArray =
@@ -68,12 +71,12 @@ public final class FileUtil {
 
     for (int i = 0; i < trackGroupArray.length; i++) {
       TrackGroup trackGroup = trackGroupArray.get(i);
-      if (trackGroup.type == C.TRACK_TYPE_VIDEO) {
-        assertThat(trackGroup.length).isEqualTo(1);
+      if (trackGroup.type == trackType) {
+        checkState(trackGroup.length == 1);
         return trackGroup.getFormat(0);
       }
     }
-    throw new IllegalStateException("Couldn't find video track");
+    throw new IllegalStateException("Couldn't find track");
   }
 
   private FileUtil() {}
diff --git a/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerEndToEndTest.java b/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerEndToEndTest.java
index 41a54f2a22..15008952f0 100644
--- a/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerEndToEndTest.java
+++ b/libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerEndToEndTest.java
@@ -17,6 +17,7 @@ package androidx.media3.transformer;
 
 import static androidx.media3.common.util.Assertions.checkNotNull;
 import static androidx.media3.common.util.Util.isRunningOnEmulator;
+import static androidx.media3.test.utils.FileUtil.retrieveTrackFormat;
 import static androidx.media3.transformer.AndroidTestUtil.JPG_ASSET_URI_STRING;
 import static androidx.media3.transformer.AndroidTestUtil.MP3_ASSET_URI_STRING;
 import static androidx.media3.transformer.AndroidTestUtil.MP4_ASSET_FORMAT;
@@ -943,13 +944,15 @@ public class TransformerEndToEndTest {
     Transformer transformer =
         new Transformer.Builder(context).setVideoMimeType(MimeTypes.VIDEO_AV1).build();
 
-    ExportResult exportResult =
+    ExportTestResult exportTestResult =
         new TransformerAndroidTestRunner.Builder(context, transformer)
             .build()
-            .run(testId, editedMediaItem)
-            .exportResult;
+            .run(testId, editedMediaItem);
+    ExportResult exportResult = exportTestResult.exportResult;
 
-    // TODO: b/322954582 - Also assert by probing output file.
+    String actualMimeType =
+        retrieveTrackFormat(context, exportTestResult.filePath, C.TRACK_TYPE_VIDEO).sampleMimeType;
+    assertThat(actualMimeType).isEqualTo(MimeTypes.VIDEO_AV1);
     assertThat(exportResult.exportException).isNull();
     assertThat(exportResult.durationMs).isGreaterThan(0);
     assertThat(exportResult.videoMimeType).isEqualTo(MimeTypes.VIDEO_AV1);
@@ -963,13 +966,15 @@ public class TransformerEndToEndTest {
     Transformer transformer =
         new Transformer.Builder(context).setAudioMimeType(MimeTypes.AUDIO_AAC).build();
 
-    ExportResult exportResult =
+    ExportTestResult exportTestResult =
         new TransformerAndroidTestRunner.Builder(context, transformer)
             .build()
-            .run(testId, editedMediaItem)
-            .exportResult;
+            .run(testId, editedMediaItem);
+    ExportResult exportResult = exportTestResult.exportResult;
 
-    // TODO: b/322954582 - Also assert by probing output file.
+    String actualMimeType =
+        retrieveTrackFormat(context, exportTestResult.filePath, C.TRACK_TYPE_AUDIO).sampleMimeType;
+    assertThat(actualMimeType).isEqualTo(MimeTypes.AUDIO_AAC);
     assertThat(exportResult.exportException).isNull();
     assertThat(exportResult.durationMs).isGreaterThan(0);
     assertThat(exportResult.audioMimeType).isEqualTo(MimeTypes.AUDIO_AAC);