fix: static file handlers shouldn't throw on unknown mime

curl https://flutter-dashboard.appspot.com/assets/shaders/ink_sparkle.frag
> null check
package:cocoon_service/src/request_handling/static_file_handler.dart:50:39

Author: jtmcdole

app_dart/lib/src/request_handling/static_file_handler.dart

@@ -45,9 +45,10 @@ final class StaticFileHandler extends RequestHandler {
     const basePath = 'build/web';
     final file = fs.file('$basePath$resultPath');
     if (file.existsSync()) {
-      final mimeType = mimeTypeMap.containsKey(path.extension(file.path))
-          ? mimeTypeMap[path.extension(file.path)]!
-          : lookupMimeType(resultPath)!;
+      final mimeType =
+          mimeTypeMap[path.extension(file.path)] ??
+          lookupMimeType(resultPath) ??
+          'application/octet-stream';
       return Response.stream(
         file.openRead().cast<Uint8List>(),
         contentType: ContentType.parse(mimeType),

app_dart/test/request_handling/static_file_handler_test.dart

@@ -108,5 +108,21 @@ void main() {
       final response = await decodeHandlerBody(body);
       expect(response, 'abc');
     });
+
+    test('Unknown extensions do not throw', () async {
+      fs.file('build/web/shader.frag').writeAsStringSync('abc');
+      final staticFileHandler = StaticFileHandler(
+        '/shader.frag',
+        config: config,
+        fs: fs,
+      );
+
+      final body = await tester.get(staticFileHandler);
+      expect(body, isNotNull);
+      final response = await decodeHandlerBody(body);
+      expect(response, 'abc');
+      expect(body.contentType, isNotNull);
+      expect(body.contentType!.mimeType, 'application/octet-stream');
+    });
   });
 }