[Glitch] Fix editing description of media uploads with custom thumbnails

Port f760899b04 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/reducers/compose.js

@@ -507,7 +507,7 @@ 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;