Fix replying to threads with empty message and a file (#4693)

webapp/components/create_comment.jsx

@@ -123,7 +123,7 @@ export default class CreateComment extends React.Component {
 
         MessageHistoryStore.storeMessageInHistory(message);
 
-        if (message.trim().length === 0 && this.state.previews.length === 0) {
+        if (message.trim().length === 0 && this.state.fileInfos.length === 0) {
             return;
         }