XBM
An image format written in C.
Where it came from
XBM (X BitMap) came from the X Window System in the late 1980s. Its bizarre feature: an XBM image is valid C source — an array of bytes you #include, and the compiler "draws" it.
That made icons trivially embeddable in X11 programs. Browsers dropped XBM support years ago, but the format lives in retro and security-curiosity circles.
How it works
An XBM is plain text: #define width 16, #define height 16, then static unsigned char bits[] = ﹛ 0x00, 0xFF, ... ﹜. Each bit is one pixel (1 = black).
Reading it is reading C. UWC converts XBM → PNG/JPEG on the server, since no browser will touch it anymore.