In the ftpagent.conf file, there is a setting for suffix. What does it represent?

Use this as an example:
file0=C:\Program Files\CiscoACS\Logs\Failed Attempts.csv
file0.suffix= active

The suffix represents the differences one might find in the naming convention. Whatever you have for the suffix line gets tagged at the end of the file name. So:
Filename= ''Failed Attempts.csv''
Suffix='' active''
Filename + Suffix= ''Failed Attempts active.csv''

(note the space at the beginning of the word active)