update extensions

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

@@ -2,14 +2,17 @@ import 'package:http_parser/http_parser.dart' as http_parser;
 import 'package:mime/mime.dart';
 
 /// Useful extension functions for [String]
-extension StringX on 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 (this!.toLowerCase().endsWith('heic')) {
+    if (toLowerCase().endsWith('heic')) {
       return http_parser.MediaType.parse('image/heic');
     } else {
-      return http_parser.MediaType.parse(lookupMimeType(this!)!);
+      final mimeType = lookupMimeType(this);
+      if (mimeType == null) {
+        return null;
+      }
+      return http_parser.MediaType.parse(mimeType);
     }
   }
 }