Beginning with 0.8.2 version Ristretto ceased to open PPM files. I was advised on the forums that this problem looks similar to another one that already has been resolved:
Thanks in advance.
Hi, is the problem still there in version 0.8.3?
Igor referenced this bugreport in commit 4d03ad59054fa48a0615ea0c055574ebe777442d
Fix opening of PPM files
This bug seems to be derived from the Ubuntu bug https://bugs.launchpad.net/ubuntu/+source/ristretto/+bug/1771227 in which case the title should have really been "PGM files are not opened".
However, regardless of that, the commit above does fix opening PGM files.