Specify allowed media in upload button tooltip. (#8443)

app/javascript/mastodon/features/compose/components/upload_button.js

@@ -7,7 +7,7 @@ import ImmutablePureComponent from 'react-immutable-pure-component';
 import ImmutablePropTypes from 'react-immutable-proptypes';
 
 const messages = defineMessages({
-  upload: { id: 'upload_button.label', defaultMessage: 'Add media' },
+  upload: { id: 'upload_button.label', defaultMessage: 'Add media (JPEG, PNG, GIF, WebM, MP4)' },
 });
 
 const makeMapStateToProps = () => {