B-3: There should be a way to identify which file is the metadata

[jcoyne]

A select list on the edit page, similar to how you identify the representative object, but with options constrained to just the xml files.

[val99erie]

Consider both initial upload and later edits

[val99erie]

Acceptance Criteria:

• For a single work, I can attach multiple metadata files (MODS, MARCXML, VRA, etc)
• In the show page for the work, the list of metadata files will appear in a separate section from all the other files.
• On the edit page, there is a way to mark/unmark the attached files as a "metadata file"