convert: no decode delegate for this image format
More testcases: Same problem for
brought up in https://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=144583567#No_thumbnails_from_pdf_files.3F_Like_File:WorldAviation.198409.BackCover.pdf
I looked into that file. File took insanely long (about 10 minutes) for ghostscript to render. My guess is ghostscript took more than the $wgMaxShellTime limit, thus gs died and sent convert something that was not a file. image magick gave that error. I don't know if there's anything we can realistically do in such a situation. We probably do not want limits being what would be required in order to render this file (Assuming I'm right in that is what is causing this).