In practice, SFTP-based client processes often use a prefix based
filter when retrieving files on the server.

ie: there is one SFTP server between 2 systems, used for different
exchanges. The SFTP client will do something like (found in the wild):
process1(file_list=sftp.list(path, prefix='PROCESS_1'))
process2(file_list=sftp.list(path, prefix='PROCESS_2'))

Our implementation of temporary_file_on_write only sets a suffix,
which doesn't work for the previous SFTP client. So let's add also a
prefix when writing temporary files.