Plugins require two kinds of files, which are loaded into the Plugins subdirectory of the Factsmith installation and are processed when Factsmith loads:
.plugin declaration files, which specify various information on how the plugin will appear in the Export dialog dropdown and the name of the .exe file to use. For more information on .plugin files, see Plugin File Format.
.exe executable files, which actually do the necessary processing.
Here's how a brief is actually exported:
.fsx..plugin file to determine what executable to use..fsx file passed as a command-line option..fsx file to create a new file in the final format, the .fsx file is deleted.For details on the format of the intermediary .fsx file, see FSX File Format and the FSX Code Reference.