Document loss of "Application (multipurpose)" or provide a way to fix this

An occasional client just updated the modules, and lost all of their functionality that was assigned to Application (multipurpose) file type.

I manually just updated the file_managed table to fix this, but was lucky that all undefined were videos, so nothing too complex was required. However, having to do this at the db layer is crazy, and would be out of the scope of many users. Even if editing the media allowed us to fix this, this task would have taken hours as there were 1,000s of files.

Marked as major as afaick, this effectively results in data loss for the average user. (The data is there, but unusable.)