I was thinking of all of the files in proprietary, and particularly binary, formats. Maybe some users know that even those files can be opened by multiple applications, when that's even true, but I suspect even more users don't even realize that almost all of their data is stored in a file somewhere, let alone
where that file is in the filesystem and in what format it's stored.
Given the ubiquity of Word document and Powerpoint presentation files and the like, most users I'll grant you are aware of the files themselves, and the fact that they can be attached to an email. I'll even grant that a large fraction of those same users could answer 'yes' to the question 'Could these files be opened by another application?'. But almost none would be capable of doing anything with those files without an application that handles everything for them.
I don't dispute tho that an awareness of, let alone existence of, files in a filesystem is a significant benefit and not having access to them is a (relatively) significant loss.