image/x-portable-pixmap

The color member of Jef Poskanzer's Netpbm family, designed in the late 1980s so that images could survive 7-bit email gateways — the ASCII P3 variant is literally numbers separated by whitespace. A PPM is the simplest possible true-color image: a two-character magic, dimensions, a max value, then pixels; you can write a parser in a dozen lines, which is why graphics courses and image-processing test suites never let it die. Nobody registered it, nobody serves it on the web, and it is gloriously inefficient — its whole value is that there is nothing to get wrong. The binary variant opens with P6, the ASCII one with P3.

.ppm is color; siblings .pgm (grayscale, P2/P5) and .pbm (bitmap, P1/P4) have their own x- types.

Defined by Netpbm PPM format specification. Registry facts from the IANA media-types registry via mime-db.