diff --git a/lib/api/route/messages.dart b/lib/api/route/messages.dart
index 618ae36c89..0345b2703b 100644
--- a/lib/api/route/messages.dart
+++ b/lib/api/route/messages.dart
@@ -262,9 +262,10 @@ Future<UploadFileResult> uploadFile(
   required Stream<List<int>> content,
   required int length,
   required String filename,
+  required String? mimeType,
 }) {
   return connection.postFileFromStream('uploadFile', UploadFileResult.fromJson, 'user_uploads',
-    content, length, filename: filename);
+    content, length, filename: filename, contentType: mimeType);
 }
 
 @JsonSerializable(fieldRename: FieldRename.snake)
diff --git a/lib/widgets/compose_box.dart b/lib/widgets/compose_box.dart
index 1e3ae73bdd..abd634dda4 100644
--- a/lib/widgets/compose_box.dart
+++ b/lib/widgets/compose_box.dart
@@ -478,7 +478,11 @@ Future<void> _uploadFiles({
     Uri? url;
     try {
       final result = await uploadFile(store.connection,
-        content: content, length: length, filename: filename);
+        content: content,
+        length: length,
+        filename: filename,
+        mimeType: null, // TODO(#829)
+      );
       url = Uri.parse(result.uri);
     } catch (e) {
       if (!context.mounted) return;
diff --git a/test/api/route/messages_test.dart b/test/api/route/messages_test.dart
index c860c4de43..b91fb3e732 100644
--- a/test/api/route/messages_test.dart
+++ b/test/api/route/messages_test.dart
@@ -414,6 +414,55 @@ void main() {
     });
   });
 
+  group('uploadFile', () {
+    Future<void> checkUploadFile(FakeApiConnection connection, {
+      required List<List<int>> content,
+      required int length,
+      required String filename,
+      required String? mimeType,
+    }) async {
+      connection.prepare(json:
+        UploadFileResult(uri: '/user_uploads/1/4e/m2A3MSqFnWRLUf9SaPzQ0Up_/$filename').toJson());
+      await uploadFile(connection,
+        content: Stream.fromIterable(content),
+        length: length,
+        filename: filename,
+        mimeType: mimeType);
+      check(connection.lastRequest).isA<http.MultipartRequest>()
+        ..method.equals('POST')
+        ..url.path.equals('/api/v1/user_uploads')
+        ..files.single.which((it) => it
+          ..field.equals('file')
+          ..length.equals(length)
+          ..filename.equals(filename)
+          ..contentType.asString.equals(mimeType ?? 'application/octet-stream')
+          ..has<Future<List<int>>>((f) => f.finalize().toBytes(), 'contents')
+            .completes((it) => it.deepEquals(content.expand((l) => l))));
+    }
+
+    test('with mime type', () {
+      return FakeApiConnection.with_((connection) async {
+        await checkUploadFile(connection,
+          content: ['asdf'.codeUnits],
+          length: 4,
+          filename: 'image.jpg',
+          mimeType: 'image/jpeg',
+        );
+      });
+    });
+
+    test('without mime type', () {
+      return FakeApiConnection.with_((connection) async {
+        await checkUploadFile(connection,
+          content: ['asdf'.codeUnits],
+          length: 4,
+          filename: 'some_file',
+          mimeType: null,
+        );
+      });
+    });
+  });
+
   group('addReaction', () {
     Future<void> checkAddReaction(FakeApiConnection connection, {
       required int messageId,