diff --git a/libraries/mediaupload/impl/src/main/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessor.kt b/libraries/mediaupload/impl/src/main/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessor.kt
index b374e530f4..eebbfbeab6 100644
--- a/libraries/mediaupload/impl/src/main/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessor.kt
+++ b/libraries/mediaupload/impl/src/main/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessor.kt
@@ -85,8 +85,9 @@ class AndroidMediaPreProcessor(
             val result = when {
                 mimeType == MimeTypes.Svg -> processSvgImage(uri, mimeType)
                 mimeType.isMimeTypeImage() -> {
-                    val shouldBeCompressed = mediaOptimizationConfig.compressImages && mimeType !in notCompressibleImageTypes
-                    processImage(uri, mimeType, shouldBeCompressed)
+                    val imageMimeType = resolveImageMimeType(uri, mimeType)
+                    val shouldBeCompressed = mediaOptimizationConfig.compressImages && imageMimeType !in notCompressibleImageTypes
+                    processImage(uri, imageMimeType, shouldBeCompressed)
                 }
                 mimeType.isMimeTypeVideo() -> processVideo(uri, mimeType, mediaOptimizationConfig.videoCompressionPreset)
                 mimeType.isMimeTypeAudio() -> processAudio(uri, mimeType)
@@ -155,6 +156,19 @@ class AndroidMediaPreProcessor(
         }
     }
 
+    /**
+     * The declared mime type can be broader than the image actually is - a wildcard from a share intent, or a mislabelled file - and the encoder is picked
+     * from it. Re-encoding a transparent image as JPEG flattens its alpha channel to black, so resolve the real format from the image header first.
+     */
+    private fun resolveImageMimeType(uri: Uri, declaredMimeType: String): String {
+        if (declaredMimeType == MimeTypes.Png || declaredMimeType == MimeTypes.Jpeg) return declaredMimeType
+        val options = BitmapFactory.Options().apply { inJustDecodeBounds = true }
+        tryOrNull {
+            contentResolver.openInputStream(uri)?.use { BitmapFactory.decodeStream(it, null, options) }
+        }
+        return options.outMimeType ?: declaredMimeType
+    }
+
     private suspend fun processImage(uri: Uri, mimeType: String, shouldBeCompressed: Boolean): MediaUploadInfo {
         Timber.d("Processing image ${uri.path.orEmpty().hash()}")
 
diff --git a/libraries/mediaupload/impl/src/test/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessorTest.kt b/libraries/mediaupload/impl/src/test/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessorTest.kt
index 3fb639aa9b..79f6fd10f8 100644
--- a/libraries/mediaupload/impl/src/test/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessorTest.kt
+++ b/libraries/mediaupload/impl/src/test/kotlin/io/element/android/libraries/mediaupload/impl/AndroidMediaPreProcessorTest.kt
@@ -70,6 +70,7 @@ class AndroidMediaPreProcessorTest : RobolectricTest() {
         mediaOptimizationConfig: MediaOptimizationConfig,
         sdkIntVersion: Int = Build.VERSION_CODES.P,
         deleteOriginal: Boolean = false,
+        mimeType: String = asset.mimeType,
     ): MediaUploadInfo {
         val context = InstrumentationRegistry.getInstrumentation().context
         val deleteCallback = lambdaRecorder<Uri?, Unit> {}
@@ -81,7 +82,7 @@ class AndroidMediaPreProcessorTest : RobolectricTest() {
         val file = getFileFromAssets(context, asset.filename)
         val result = sut.process(
             uri = file.toUri(),
-            mimeType = asset.mimeType,
+            mimeType = mimeType,
             deleteOriginal = deleteOriginal,
             mediaOptimizationConfig = mediaOptimizationConfig,
         )
@@ -91,6 +92,52 @@ class AndroidMediaPreProcessorTest : RobolectricTest() {
         return data
     }
 
+    @Test
+    fun `a png shared with a wildcard mime type is still encoded as a png`() = runTest {
+        val mediaUploadInfo = process(
+            asset = assetImagePng,
+            mimeType = MimeTypes.Images,
+            mediaOptimizationConfig = MediaOptimizationConfig(
+                compressImages = true,
+                videoCompressionPreset = VideoCompressionPreset.STANDARD,
+            ),
+            sdkIntVersion = Build.VERSION_CODES.Q,
+        )
+        val info = mediaUploadInfo as MediaUploadInfo.Image
+        assertThat(info.imageInfo.mimetype).isEqualTo(MimeTypes.Png)
+    }
+
+    @Test
+    fun `a gif shared with a wildcard mime type is recognised and left uncompressed`() = runTest {
+        val mediaUploadInfo = process(
+            asset = assetAnimatedGif,
+            mimeType = MimeTypes.Images,
+            mediaOptimizationConfig = MediaOptimizationConfig(
+                compressImages = true,
+                videoCompressionPreset = VideoCompressionPreset.STANDARD,
+            ),
+            sdkIntVersion = Build.VERSION_CODES.Q,
+        )
+        val info = mediaUploadInfo as MediaUploadInfo.Image
+        assertThat(info.imageInfo.mimetype).isEqualTo(MimeTypes.Gif)
+        assertThat(info.imageInfo.size).isEqualTo(assetAnimatedGif.size)
+    }
+
+    @Test
+    fun `a file that is not an image keeps the wildcard mime type it was shared with`() = runTest {
+        val mediaUploadInfo = process(
+            asset = assetText,
+            mimeType = MimeTypes.Images,
+            mediaOptimizationConfig = MediaOptimizationConfig(
+                compressImages = false,
+                videoCompressionPreset = VideoCompressionPreset.STANDARD,
+            ),
+            sdkIntVersion = Build.VERSION_CODES.Q,
+        )
+        val info = mediaUploadInfo as MediaUploadInfo.Image
+        assertThat(info.imageInfo.mimetype).isEqualTo(MimeTypes.Images)
+    }
+
     @Test
     @Ignore("Ignore now that min API for enterprise is 33")
     fun `test processing png`() = runTest {