[Async Attachment Upload] Initial implementation

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat/lib/src/extensions/string_extension.dart

@@ -0,0 +1,15 @@
+import 'package:http_parser/http_parser.dart' as http_parser;
+import 'package:mime/mime.dart';
+
+/// Useful extension functions for [String]
+extension StringX on String {
+  /// Returns the mime type from the passed file name.
+  http_parser.MediaType get mimeType {
+    if (this == null) return null;
+    if (toLowerCase().endsWith('heic')) {
+      return http_parser.MediaType.parse('image/heic');
+    } else {
+      return http_parser.MediaType.parse(lookupMimeType(this));
+    }
+  }
+}