Fix editing description of media uploads with custom thumbnails (#32221)

app/javascript/mastodon/reducers/compose.js

@@ -402,7 +402,7 @@ export default function compose(state = initialState, action) {
       .set('isUploadingThumbnail', false)
       .update('media_attachments', list => list.map(item => {
         if (item.get('id') === action.media.id) {
-          return fromJS(action.media);
+          return fromJS(action.media).set('unattached', item.get('unattached'));
         }
 
         return item;