What we are trying to understand is, first and foremost, if there are known cases under which PostgreSQL can truncate a file.
I think it's somewhat more likely that whatever is sending the file to PG is the cause, either in how it handles the file or due to communications issues.
This sounds similar to a problem I experienced with an application I wrote that takes files extracted from email (using MHonarc) and stores them in a PG database so that I can render them using a web browser later on. I wound up having to store the files in BASE64 encoding to keep them from getting corrupted. -- Mike Nolan