fix: incorrect mimeType for files when downloading

lib/src/event.dart

@@ -792,7 +792,11 @@ class Event extends MatrixEvent {
         throw ('Unable to decrypt file');
       }
     }
-    return MatrixFile(bytes: uint8list, name: body);
+    return MatrixFile(
+      bytes: uint8list,
+      name: body,
+      mimeType: attachmentMimetype,
+    );
   }
 
   /// Returns if this is a known event type.

lib/src/utils/matrix_file.dart

@@ -41,8 +41,9 @@ class MatrixFile {
   }
 
   MatrixFile({required this.bytes, required String name, String? mimeType})
-      : mimeType = mimeType ??
-            lookupMimeType(name, headerBytes: bytes) ??
+      : mimeType = mimeType != null && mimeType.isNotEmpty
+            ? mimeType
+            : lookupMimeType(name, headerBytes: bytes) ??
                 'application/octet-stream',
         name = name.split('/').last;