There is only one file type that uses the .MXMF extension:
Mobile XMF Ringtone File
Audio file created in the Mobile XMF (Extensible Music Format) format, an open standard audio format used for storing mobile phone ringtones; may contain both MIDI data as well as Mobile DLS (Downloadable Sounds) wavetable synthesized audio.
The MXMF format enables both recorded audio and MIDI data to be stored within a compact file size, which is suitable for ringtones music.
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Updated: April 2, 2011