Search for mid to rtttl converter or software able to handle these file types.
Convert MIDI audio to RTTTL / Nokring mobile ring-tone format.
Plenty of old and somewhat obsolete audio files can be still opened in programs like Awave Studio that supports many formats and can both edit and convert them. So if you are looking for mid to rtttl conversion, your best option would be to use this program.
Files with rtttl suffix can be found as old ringtones in NokRing format used in Nokia phones.