Subdir option for m2h_external::filter not working

Bug discovery

The issue is present in the latest release.
I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Activating the Subdir and usename for m2h_external::filter seems to provoke a bug. MhonArc fails to create the subdirs.

Expected Behavior 🤔

MhonArc should to create the subdirs for each mail.

Steps to Reproduce 🕹️

Steps:

Add Subdir and usename to m2h_external::filter
Try to generate the views using the template
See errors

Context 🔦

This feature can be useful to people that wants to conserve the filename of the attached files, and to sort out every file to know which files belongs to which message.

Your Environment 🌎

Env infos	Version
Ubuntu	20.04