Ok, I think I have a working patch for this issue. I patched 
ido-file-internal, ido-read-file-name, and ido-read-directory-name, and 
I think that's all the code paths that need to be fixed. Now the test 
case described in the original report produces the correct result for 
me: writing to /tmp/mumble.frotz.

However, I should note that this is still relatively untested. I will 
test it out and try to make sure it doesn't cause any unexpected issues 
before I recommend merging it.