Are you using GarageBand on a Mac or on an iPad?
Assuming you are using a Mac with GarageBand '11 on MacOS 10.6.8 or 10.7.3:
GarageBand can play many kinds of midi, but not all - you just will have to try. The same goes for iTunes. If you import midi to iTunes, then iTunes will play it, and keep it midi, but it will not show up when searching for the "kind is music". You will find the song at the top of the "recently added" playlist.
To open a midi sent by Mail in GarageBand, press the "Save" drop-down menu button in the Attachment section of the Mail Window. Save the midi-attachment to the Desktop. Then try to import it to a GarageBand Project by dragging the midi file to an empty Software Instrument track.