| 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 |
| mcxe | MedicinesComplete Data |
| 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 | Notation Composer Midi Device Description |
| mdd | Xilinx Microprocessor Driver Description |
| mddata | iPhone Backup |
| mdf | Microsoft SQL Server Database |
| mdg | myPM Merge Data |
| mdif | EEsof Model MDIF File |
| mdinfo | iPhone Backup Information |
| mdl | STAR-CD v4 Model Data |
| mdm | BrainVoyager QX MDM Data |
| mdm | HLM Multivariate Data Matrix |
