Support audio attachments

Mastodon 2.9.2 now supports native audio attachments, so
any mp3 enclosures detected from a podcast-style feed
are converted to an audio attachment of the form
"attachment": {
  "type": "Document",
  "mediaType": "audio/mpeg",
  "url": "[URL of the mp3]",
  "name": null
}

updateFeeds.js

@@ -223,7 +223,16 @@ function createMessage(text, name, domain, item, follower, guidNote) {
   // add image attachment
   let attachment;
   console.log('NUM IMAGES',item.urls.length);
-  if (item.urls.length > 0) {
+  if (item.enclosure && item.enclosure.url && item.enclosure.url.includes('.mp3')) {
+    attachment = {
+      'type': 'Document',
+      'mediaType': 'audio/mpeg',
+      'url': item.enclosure.url,
+      'name': null
+    };
+    out.object.attachment = attachment;
+  }
+  else if (item.urls.length > 0) {
     attachment = {
       'type': 'Document',
       'mediaType': 'image/png', // TODO: update the mediaType to match jpeg,gif,etc