.SEPLUGIN File Extension
File TypeAppleScript Editor Plug-in
What is a SEPLUGIN file?
Plug-in file used by AppleScript Editor, an AppleScript program included with Mac OS X; contains functionality that extends the editor; may be a pre-installed add-on or a custom add-on created by the user.
SEPLUGIN files are actually bundled folders with the ".seplugin" extension. You can view their contents by removing the extension and opening the resulting folder.
Some SEPLUGIN files are included in the AppleScript Editor application bundle in the /Contents/Plugins/ directory.