Triggers are only executed when an event happens. (Successful upload, login unsuccessful, etc). It doesn't "monitor" a folder. I mention this as I don't want users to be misled.
If you have a Trigger defined looking for files to be uploaded to your system (which I believe is the situation you are describing) then this is triggered based on the successful upload of a file to your system. The issue is being identified by the ".filepart".
The "event" that is happening is that "ABC123.CSV.filepart" is being uploaded. By time the trigger is responding, the source system is renaming the "ABC123.CSV.filepart" to "ABC123.CSV"
If this is the case, you have a couple of things to consider.
1- Ask your trading partner to NOT send the file with ".filepart" (This is a setting on their software)
2- When your trigger event is activated and you're processing the file through a WorkFlow, just "assume" the ".filepart" isn't there. (remove it from the file name you're looking for) then process the "ABC123.CSV".
3- When the trigger is activated, generate a filelist of the folder from the WorkFlow ... which should not find the "ABC123.CSV.filepart" file ... then process what it finds.