Hi Brandon.
No, you didn't overlook anything, but I'd say we did. This is a bug.
We can get it fixed in version 17.0 (due out next month). If you are on version 16.3, you can get a fix for this in beta on Monday (Sept 25)
Maybe as a workaround you can add a RExec action first to rename (or copy) the file to a name that the FTP action can expect? Looks like variables in the RExec command are processed as expected, so I believe this will work, although I did not test it.
-John