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]]
+
+>> 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]]
+
+>>> Ok, so it's not removal specific, can in fact be triggered by using
+>>> testpagespec (or really anything besides attachment, which passes
+>>> the filename parameter). Nor is it limited to mimetype, all the tests in
+>>> filecheck have the problem. [[Fixed|done]] --[[Joey]]
+
The following patch fixes the problem:
diff --git a/IkiWiki/Plugin/filecheck.pm b/IkiWiki/Plugin/filecheck.pm