FTI stands for Flat Transparency Image.

Flat – it's a plain 2D image format, no layers or vector data.
Transparency – it stores a full alpha channel, so parts of the image can be see-through.
Image – it's an image format, holding pixel data.

Like JFIF, an .FTI file is small and simple, but unlike JFIF it keeps a real transparent background instead of a solid one.

For a file menu and more features, download FTI Studio.