Dear Gnus,

If you go to the attachment of this post and hit 'E', assuming that your
Gnus doesn't know how to handle mimetype "application/octet-stream", you
will then be prompted in minibuffer:

  View as MIME Type (default application/octet-stream):

Now if you press any key an error message will be displayed:

  Error in post-command-hook: (wrong-type-argument stringp (text/xml))

And the biggest inconvenience is that it does not do completing-reading
at all.