A much earlier version did support this common file format, but for some strange reason it was removed in 2.x.
A pgm/ppm file is a wonderfully simple format - it is basically a simple textual header followed by a binary dump of the image data. So it is trivially easy to parse and load or save. It is for this reason that it is so useful, for example when working on image processing algorithms, it is so convenient to export test data, intermediate results and so forth.
I hope they will (re-)add support for pgm/ppm files soon. If you think this would be useful too, please vote on the support site.