Ability to associate media files from rewriten img tags into another media field.

Use case:

On the destination content type, you have a bidy field, and a multivalue media field. The idea being that when you embed medi into a node, you also associate that media with the node through this media field. When #1062304: Fill in the available fields when creating media in the WYSIWYG gets completed, this will become really simple for the user, as they can just add media to their node through the WYSIWYG, and have it automatically associated with the node by means of a media field like the one on this content type I'm describing. Also useful with Media Insert - http://drupal.org/sandbox/sreynen/1141496 - which should also become part of media soon enough #1140404: Add insert module integration.

So, migrate_extras can strip out embedded img tags and replace with media embeds that reference managed files. In order to service the above use case, we would need a be able to define a 'target media field', so that after replacing the img tag with the media embed, the fid of the managed file in question can be added to the target media field.

The end result is that you end up with managed files nicely embedded into the content using a media embed, and you get the managed file properly associated with the node through a media field.