File extensions that begin with M.

 Ext.  Type of file
 M1V   MPEG movie
 M3D   Corel Motion 3D
 M3U   Playlist
 M4A   AAC - MPEG-4 audio
 M4B   AAC - MPEG-4 audiobook (protected)
 M4P   AAC - MPEG-4 audio (protected)

 MAC   MacPaint bitmap
 MAK   Visual Basic/MS C++ project
 MAP   Link editor map

 MB1   Apogee Monster Bash
 MBX   Mailbox (e-mail)

 MCS   MathCAD format
 MCW   Word for Macintosh document

 MDB   Access database

 MEL   Maya script
 MET   OS/2 Metafile
 MEU   Menu items

 MGX   Micrografx clip art

 MDX   dBASE IV multi-index

 MID   MIDI sound
 MID   Eudora script
 MIL   Same as GP4
 MIX   PhotoDraw

 MME   MIME-encoded
 MMF   Microsoft mail
 MMM   Macromind animation format

 MOD   Eudora script
 MOV   QuickTime movie

 MPA   MPEG
 MP2   MPEG
 MP2V  MPEG
 MP3   MPEG-1 Layer 3 audio
 MP4   MPEG-4 file
 MPE   MPEG
 MPEG  MPEG
 MPG   MPEG
 MPP   Microsoft Project

 MRK   Informative Graphics markup
 MRW   Minolta RAW

 MSC   Microsoft Management Console snap-in
 MSG   Message
 MSI   Microsoft installation package
 MSO   (oledata.mso) Rendering info for
        Word documents attached in e-mail
 MSP   Microsoft Paint bitmap

 MUS   Music

 MVB   Microsoft Multimedia Viewer

 MXF   Material Exchange Format (Pro-MPEG Forum)
 MXF   Home Theater Master remote control