Hi Eric,
Where you select "output to file system" so that SAPFPAYM program generate a file to AL11, it create a text file and normally add suffix of 4 digit number (last 4 digits of EDINUM) and file extension like "csv" may not work.
So the question is what middleware do you use to move the files from your file system (AL11) to Banks? If it is XI/PI, you might consider adding a file extenstion at middleware.
Hope that this helps
Cheers
Taro