Comment 1 for bug 505880

This is correct; the code will skip files which start with a dot, or if it contains a character which is not alpha numeric or "-" or "_"; it will also only consider regular files for some reason (ignoring e.g. symbolic links and subdirs).

This is supposed to be some run-parts-alike filter mechanism to avoid reading editor backup files, or files leftover by the dpkg conffile mechanism.

However, this should be documented in the man page.

See the sources.list man page and the section on .list files for some example filenames (ignore the requirement of files ending in .list).