use RSS's <enclosure> element instead of <wxr:attachment_url>

[pbiron]

The RSS Advisory Board's "Best Practices" profile says, in part

When a namespace element duplicates the functionality of an element defined in RSS, the core element should be used.

and the semantics of RSS's exactly matches the semantics of <wxr:attachment_url>.

RSS's <enclosure> has 3 required attributes:

• The length attribute indicates the size of the file in bytes
• The type attribute identifies the file's MIME media type
• The url attribute identifies the URL of the file

The file size and mime-type is available at the time an export is generated (and of course, so is the URL), so there would be no problem with using <enclosure> in newly generated WXR instances.

Making this change would slightly complicate matters for the transform from WXR 1.0, 1.1 and 1.2 instances into WXR 1.3-proposed instances that is part of the WordPress Importer Redux. However, the complication is only slight, since the response headers returned by a call to wp_remote_head() on the URL SHOULD provide the necessary information.