It turns out that the filecheck plugin couldn't find the file, because it was merely using the $pagesources hash, rather than finding the absolute path of the file in question.
> I don't understand why the file was not in `%pagesources`. Do you?
-> --[[Joey]]
+> --[[Joey]]
+
+>> The file *was* in `%pagesources`, but what returns from that is the filename relative to the `srcdir` directory; for example, `foo/bar.gif`.
+>> When File::MimeInfo::Magic::magic is given that, it can't find the file.
+>> But if it is given `/path/to/srcdir/foo/bar.gif` instead, then it *can* find the file, and returns the mime-type correctly.
+>> --[[KathrynAndersen]]
The following patch fixes the problem: