| mbx | Eudora Mailbox File |
| mbx | Microsoft Outlook Express v4 Mailbox |
| mc1 | MedCalc Data |
| mc3 | MC3D Document |
| mc4 | MicroCase v4 Data File |
| mc4d | Maxon Cinema 4D Object |
| mc5 | Poser 5 Material Collection |
| mc6 | Poser 6 Material Collection |
| mcb | NEPLAN Root Network Data |
| mcdb | Media Cache Database |
| mcf | MPRESS Control Format File |
| mcfi | Autodesk Maya Initial Fluid Cache |
| mcfp | Autodesk Maya Fluid Cache Playback |
| mcif | Macromolecular CIF File |
| mck | Macro Keys Encrypted Macro |
| mcl | SmartCAM Macro File |
| mcmac | MasterCook Mac Document |
| mcml | Windows Media Center Markup Language File |
| mcr | General CADD Macro |
| mcs | Maximal Compressed Structure |
| mcs | Intel MCS-86 Hexadecimal Object |
| mcsv | TouchGraph Navigator MCSV File |
| mct | Live Messenger Content |
| mcv | ManyCam Effect |
| mcx | MasterCook 5 Recipe Pictures |
| mcz | Poser Compressed Material Collection |
| md | Chaos Desktop Metadata |
| md | Sega Genesis ROM MD |
| md5 | MD5 Checksum |
| md5sum | MD5 Checksum |
| md5sums | MD5 Checksum |
| mdat | REW Measurement Data |
| mdb | RockWorks Borehole Database |
| mdbx | Microsoft Access JET Database |
| mdc | Cognos Cube File |
