Below described are the file formats used in the Infinity Ward engine. and methods to convert, extract and repack files.
Fast Files are file containers. Maps and Mods are packed in this format. Some unpacking and repacking tools do exist. The fast file unpacker by Tom Crowley works well for unpacking gsc scripts.
Infinity Ward Directory Files
Another container file format. IWD files are simply ZIP files and can be opened and written with any program understanding the ZIP file format. 7-zip is a good choice to edit IWD files because it lets you directly edit contained files.
Infinity Ward Image Files
Proprietary image format. Can be converted from and to with iwi2dds (DirectDraw Surface) and dds2iwi tools. Images in DDS file format can further be edited comfortably in photoshop using the Nvidia Texture Tools.
Text files containing server side scripting code. As GSC syntax is quite similar to C, syntax highlighting more or less works in all editors. The best known solution currently is using Visual Studio Code with the CoD-Sense plugin, which provides syntax highlighting explicitely for GSC code.
Textfiles containing server commands line by line.