allowing for two files in the template

[djarecka]

right now pydra will raise an error, because I didn't now what to do with the extensions of multiple file.

We should allow to have multiple files, but care only about the extension from the first field, and removing extensions from other fields.
You can always enforce the specific extension by including it in the template (in this case all extensions from the input fields will be ignored)

based on the discussions with @effigies