I had a conversation in #bugs on irc.gnome.org found here: http://pastebin.com/849858 When I rename the .img to .iso, everything works fine.
We can't have magic for ISO images, as the magic data is too far into the file, and would require reading 32k of data from the file before it can be asserted whether it's an ISO image or not.
*** Bug 55253 has been marked as a duplicate of this bug. ***
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