The format of the files is very close to that of the Memory Tables. Each file corresponds to a top-level entry in %Props. It contains a list of property tables, one for each page that contains this property -- those tables are separated by $FS1. The keys are delimited from their values by $FS2. And the individual values are delimited by $FS3.
Note that these files are the source of the one major inefficiency in ProWiki: since all values for a given property are put into one file, it means that, in a highly-structured database, a large fraction of the DB could wind up in a single file. This is high on my ToDo List, to reduce this inefficiency.