Change audio format from ogg to mp3 for wider compatibility (#11141)

* Change audio format from ogg to mp3 for wider compatibility

* Add media description as title to links of unknown media attachments

app/javascript/styles/mastodon/components.scss

@@ -557,6 +557,7 @@
 
     .compose-form__upload-thumbnail {
       border-radius: 4px;
+      background-color: $base-shadow-color;
       background-position: center;
       background-size: cover;
       background-repeat: no-repeat;