Partly transferred files are stored with @file{.part} suffix. And
possibly they also require cleanup if they are completely lost.
+ It can contain @file{.nock} files: fully downloaded, but still not
+ checksummed. Can be checksummed (with @file{.nock} extension removing)
+ with @ref{nncp-check -nock}.
+
Also it can contain @file{.seen} files, that should be cleaned too
from time to time.