| mcs | Intel MCS-86 Hexadecimal Object |
| mcs | Maximal Compressed Structure |
| mcsv | TouchGraph Navigator MCSV File |
| mct | Live Messenger Content |
| mcu | Nokia DCT4 MCU Firmware Flash Data |
| mcv | ManyCam Effect |
| mcx | MasterCook 5 Recipe Pictures |
| mcxe | MedicinesComplete Data |
| mcz | Poser Compressed Material Collection |
| md | Chaos Desktop Metadata |
| md | Sega Genesis ROM MD |
| md03 | Macromedia Director Data |
| md2p | MessengerDiscovery M2DP Data |
| md2s | MessengerDiscovery Data |
| md5 | MD5 Checksum |
| md5sum | MD5 Checksum |
| md5sums | MD5 Checksum |
| md93 | Macromedia Director 4 Data |
| md95 | Adobe Director for Mac Data |
| mdat | REW Measurement Data |
| mdb | Nastran Modal Database |
| mdb | RockWorks Borehole Database |
| mdbx | Microsoft Access JET Database |
| mdc | Cognos Cube File |
| mdccache | Microsoft Document Connection Cache Data |
| mdcs | MATLAB Distributed Computing Server Saved Session |
| mdd | MDict Resource Data |
| mdd | Notation Composer Midi Device Description |
| mdd | Quanquest SPSS Questionnaire Definition Data |
| mdd | Xilinx Microprocessor Driver Description |
