| 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 | Sega Genesis ROM MD |
| md | Chaos Desktop Metadata |
| 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 |
| mdcs | MATLAB Distributed Computing Server Saved Session |
| mdd | Xilinx Microprocessor Driver Description |
| mdd | Notation Composer Midi Device Description |
| mdf | Microsoft SQL Server Database |
| mdg | myPM Merge Data |
| mdif | EEsof Model MDIF File |
| mdl | STAR-CD v4 Model Data |
| mdm | BrainVoyager QX MDM Data |
| mdm | HLM Multivariate Data Matrix |
| mdm | OvenWATCH Dataset |
| mdn | Microsoft Access Blank Database Template |
| mdo | Microsoft IIS Metabase Backup |
| mdp | GROMACS Molecular Dynamics Program |
| mdr | ZX Spectrum Emulator Microdrive Cartridge Data |
| mdsx | MEGA Saved Session |
| mdu | D-Flow FM Model Definition Unstructured Data |
| mdw | Microsoft Access Workgroup Data |
| mdx | dBASE IV Multiple Index |
