Revisions and Commits
Turns out that this file starts with some invisible characters before the <?xml, which makes it fall back to the default attempt of rendering through the imagemagick engine - not knowing what kind of file we're dealing with - and that in turns fails in this particular way (but would fail later by being unable to convert the image anyway).
The invisible characters are simply UTF-8 encoded BOM. Which means that this might be a valid file still and that's the issue to solve. It's probably a good idea to skip the BOM in general, for all file types.