feat(route): Include enclosures for mp3, mp4, and m4a files attached to kemono posts (#13821)

* Include enclosures for mp3 and mp4 files in kemono posts.

* Don't engage with attachment links that don't have a href.

* Use the attachment's download attribute to derive a filename.

This way, kemono can change the query string in its URLs without
breaking how we're deriving relevance and mime type.

* Accept m4a as a type of audio enclosure.

lib/v2/kemono/index.js

@@ -92,6 +92,26 @@ module.exports = async (ctx) => {
                         item.guid = item.link.replace('kemono.su', 'kemono.party');
                         item.pubDate = parseDate(content('.timestamp').attr('datetime'));
 
+                        // find the first attachment with a file extension we
+                        // want, and set it as the enclosure
+                        content('.post__attachment-link[href][download]').each(function () {
+                            const extension = content(this).attr('download').replace(/.*\./, '').toLowerCase();
+                            const mimeType =
+                                {
+                                    m4a: 'audio/mp4',
+                                    mp3: 'audio/mpeg',
+                                    mp4: 'video/mp4',
+                                }[extension] || null;
+                            if (mimeType === null) {
+                                return;
+                            }
+
+                            item.enclosure_url = content(this).attr('href');
+                            item.enclosure_type = mimeType;
+
+                            return false;
+                        });
+
                         return item;
                     })
                 )