The regex for the internal file test is broken - it does not allow for directory separators or spaces in the filename. The only regex that does is the internal menu path, but it does not allow "." characters. Hence something like "sites/default/files/my great presentation.pdf" fails all URL validation tests even though the file exists.
I also discovered that the internal menu path regex also doesn't allow spaces (which are legit, and both the URL aliases module and core menu router allows them) so added that character there as well.