lost default file types upgrading from unstable7 to alpha2

The upgrade code for this seems to have been introduced in #2067283: Remove the obsolete file_extensions media variable.

Upon updating from unstable7 to alpha2 we note that our custom file types for the media browser plugin in the wysiwyg are lost.... 'file_entity_default_allowed_extensions' was not populated with anything. We're getting around this by just setting 'Default allowed file extensions' on 'admin/config/media/file-settings' manually.

I've still got unstable7 in production and am running some devel to figure out how/why it failed. On our prod site, 'file_entity_default_allowed_extensions' is not set to anything and 'media__file_extensions' was gif, jpg, png & jpeg.... It seems the array_diff returns 0 items because the $media_file_extensions is a subset of those in $file_entity_file_extensions.... as such, nothing gets set for the file entity variable & the values from the media variable are lost.

For our site, (one without anything set for file_entity), it would have sufficed to just set 'file_entity_default_allowed_extensions' to what was present in 'media__file_extensions'