trying to send with a parameterized sourcefile doesn't send the file

I have a parameter with paths to the correct files and want to loop over them. When using the hardcoded path, there's no problem but once I start replacing the path with variables the file is no longer sent along.
I've used the "filename" to check for the validity of the path but couldn't spot any mistakes here.
Could you maybe help me? Do you need any more information?