[FEATURE] Keep original filenames of uploads in `{powermail_all}` marker

[kitzberger]

A user might find it odd that the name of the file that they've just uploaded got changed due to the randomizeFileName setting. Therefore I suggest to keep the original filename for each uploaded file to be used when rendering the {powermail_all} marker

Idea originally mentioned here: https://github.com/in2code-de/powermail/pull/965