Allow media to be selected based on mime type

Problem/Motivation

The WordPress source may have multiple types of media but there is no way to write migrations to different media types.

Steps to reproduce

N/A

Proposed resolution

Add a configuration option to allow users to select the MIME type the migration should return. The users can then write multiple migration for each media type they wish to migrate.

Remaining tasks

Do it

User interface changes

None

API changes

New configuration option added on WpMedia

Data model changes

None