This page contains a comprehensive list of the different file types used by the M.U.G.E.N engine and any additional content such as characters, stages and screenpacks. Please note that extensions for text-based files can be substituted with any other extension, as M.U.G.E.N is able to read any extension as long as there's unformatted text inside the file; as such, some creators may choose to not use the default extensions at all, with the exception of the .def file, which has to remain as such.
The definitions file (.def) is the most commonly used file in the M.U.G.E.N engine, as it is required for any content to function properly.
This file tells the engine what files the content uses and where they are located, what characters to be included on the select screen and their locations, what stages to be included, what stages the characters are to be fought on in Arcade mode, what order they appear in and how many can be fought per order, etc.
The sprites file (.sff) contains every image (sprite) used by a stage, character, screenpack, etc. Because M.U.G.E.N runs off indexed images with 256 colour palettes, transparency has to be a single forced colour rather than actual transparency. If a sprite isn't indexed, it runs off the palette of the computer's operating system, with the default transparency colour being black.
The .sff files received a new version coupled with the release of M.U.G.E.N 1.0 known as sffv2, which can integrate the functionalities of the .act files, being used in characters such as Kung Fu Man 720 and Kamekaze's Captain Falcon. Ssfv2, in turn, received another version with the release of M.U.G.E.N 1.1, denominated ssfv2.1, which supports 24bit and 32bit images, as well as being capable of displaying RGBA images (if RenderMode is set to OpenGL in the configurations file). Neither of these .sff versions can be opened with the old versions of Fighter Factory.
The animations file (.air) plays a major role in any character, as without it, the character would not actually appear on-screen. It determines what sprites are used for each action a character does, what order they appear in, alpha transparency, the rotation of the sprites in the animation, the hit/collision boxes, and how fast the animations play.
Although .air files are commonly associated with characters, they are also used by the M.U.G.E.N engine to display in-game effects like hitsparks, dust and SuperPauses, however, a character can have its own effects in its .air file. The only effect that will always appear is the dust that's created when a character lands from a jump.
Sound files (.snd) contain the sound effects produced by a character, screenpack, or lifebar. Only .wav files are supported, and once imported, are put into user-chosen groups that can be accessed via the PlaySnd state controller (characters), or certain pieces of code such as
cursor.move.snd = 100,0 (screenpacks) or
round1.snd = 0,0 (lifebars).
Backup files (.bak) are generated by Fighter Factory when the button to create them is clicked (yellow padlock with a white background) and as such can only be read by Fighter Factory. They serve as backups of all the files that were open at the time when the button was clicked and are useful when creating any content for M.U.G.E.N if a disaster occurs and all the original files become lost or corrupted.
Palette files (.act) are the 256 colour data files that determine what colours go where on a character. A single M.U.G.E.N character can have up to 12 act files that can be read by the M.U.G.E.N engine - these are determined by the .def file.
The constants and state files (usually .cns and .st) are highly important files that contain all of the character's coding; without them, a character would be unable to function. Every single action a character does (including the idle animation) is programmed into these sets of files along with any subsequent coding like damage and the actual frames a move can land hits on (providing the frames have red hitboxes).
The command file (.cmd) is the way to configure how a character's moves are activated, as well as any A.I. programming it may have. A.I. programmed move activations use separate codes that should not be possible for a human to input, so as to avoid the engine getting confused between a human player and a CPU player. If A.I. programming isn't set through the .cmd file, then the character (when controlled by the CPU) will run off the engine's default A.I. programming instead.
While most text-based file formats can be easily edited in word processing applications, .cmd is a file extension used by most Windows operating systems to denote a Windows Command Script, which cannot be opened by anything other than what Windows has assigned to it without opening it from within a program. Additionally, performing certain system operations such as Windows Restore will cause Windows to overwrite any file with a .cmd extension as part of its backup & restore routines; this is due to the fact that Windows cannot distinguish whether or not the file in question is an actual Windows Command Script or a file to be used by the M.U.G.E.N engine. Because of this issue, some creators will change the file extension to something else as a preventive measure (common examples include .cma and .cmnd).
The configuration file (.cfg) is used to configure and tweak the different settings of the M.U.G.E.N engine, such as screen size and resolution, any additional plugins, and whether the A.I. can 'cheat' or not; if A.I. Cheat is set to 0, then the A.I. has to activate attacks like a player would.
Font files (.fnt) are used by the M.U.G.E.N engine to determine what the in-game text will look like.