This looks like a bug. Using v1.4.1 if I click the Edit link for a previously uploaded file in an Item's File add view I'm taken to a new browser page page which includes at the bottom of Item metadata a new set of legacy metadata. This is the only (?) place where this legacy metadata is exposed.
When you click the Edit (or show) link for a file, you're taken to a page for the file's metadata, which is separate from the Item metadata.
The "Legacy" element set that only appears for files is there to account for very old Omeka sites that used to store more information directly in the Files table. It won't be there when editing or viewing an Item, only a File.