[luatex] Prevent stop when tiff file loads

[Arthur Reutenauer]

> https://bitbucket.org/leafstorm/lua-mimetypes

  I'm sorry Paulo, but this is only mapping file extensions to MIME
types, it doesn't try to guess the file type from its actual contents;
and here as I understand Patrick's files don't have an extension at all,
so emulating file(1), in at least a rudimentary fashion, is the only
option.

	Best,

		Arthur