reset post attachments on post prop change (#7158)

webapp/components/post_view/post_body_additional_content.jsx

@@ -78,6 +78,7 @@ export default class PostBodyAdditionalContent extends React.PureComponent {
         return (
             <PostAttachmentList
                 attachments={attachments}
+                key={this.props.post.id}
             />
         );
     }