Filter auto enclosure links before processing

[niallkennedy]

​The do_enclose() function looks through post content for enclosure-worthy URLs, requests the URLs it finds, and stores in post meta.

I use sample code and dummy URLs in my posts: e.g. http://example.com/song.mp3. Right now do_enclose() finds the mention of the MP3, adds it to post meta, which later outputs in rss_enclosure() or atom_enclosure(). If I want to remove the enclosure from final markup I can attach to the added_postmeta action to remove a value after it was requested and inserted or act on the markup generated by rss_enclosure() or atom_enclosure() by emptying the string.

I would like to filter out the auto-discovered enclosure link before it is processed by WordPress, looked up in the database, and HEAD requested. If a filter existed on the parsed enclosure links I could remove links from dummy domains such as "example.com."

[niallkennedy]

apply_filters on post links

[nacin]

Similar request: #18506

[SergeyBiryukov]

Related: #14493

[SergeyBiryukov]

New filters should be documented as per the ​documentation standards.

[stevenkword]

Doc added

[DrewAPicture]

In 35288:

Media: Introduce the enclosure_links filter, which makes it possible to adjust the list of audio and video enclosure links derived from post content before querying the database.

Props niallkennedy, stevenkword.
Fixes #19890.