Add additional file signature check to ensure proper staging

Add additional file signature check to ensure proper staging

There is a (very remote) possibility of the uploaded file being corrupted
while transferring it from the scratch directory to the staging directory,
since this transfer can actually copy data instead of being a simple
rename(2) call.  This commit adds a safeguard against this risk by
rechecking the detached signature after the file pair is moved.

The testsuite is updated accordingly.