REST: A typo caused a compilation error in the /REST/1.0/attachment/NN endpoint

Uncaught since 411c37f when it was originally contributed via a patch in 2008.
REST: Send a proper MIME type when returning a non-text attachment

Mimics the behaviour of /REST/1.0/ticket/X/attachments/Y/content, of
which /REST/1.0/attachment/NN/content is mostly a copy.
Merge branch '4.0/rest-attachment-compilation-error' into 4.0-trunk