RSS enclosures are a way of attaching multimedia content to RSSfeeds by providing the URL of a file associated with an entry, such as an MP3 file to a music recommendation or a photo to a diary entry. Unlike e-mail attachments, enclosures are merely hyperlinks to files, the actual data are not embedded into the feed. Support and implementation among aggregators varies: if the software understands the specified file format, it may automatically download and display the content, otherwise provide a link to it or silently ignore it.
The addition of enclosures to RSS, as first implemented by Dave Winer in late 2000http://backend.userland.com/rss092, was an important prerequisite for the emergence of podcasting, arguably the most common use of the feature as of 2006. In podcasts and related technologies enclosures are not merely attachments to entries, but provide the main content of a feed.
In RSS 2.0, the syntax for the <enclosure> tag, an optional child of the <item> element, is as follows:
<enclosure url="http://example.com/file.mp3" length="123456789" type="audio/mpeg" />
where the value of the url attribute is a URL of a file, length is its size in bytes, and type its mime type.
There may only be a single <enclosure> per <item>.
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