[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: #965

[tntrpsw]

Additionally to this feature request I would like to ask to not only use the supported filenames within templates, but also for the files being attached to emails sent out by powermail.

That would make things much easier, at least for some of our customers (website owners).

Maybe it could be done by extending the attachFromPath() function call within the addAttachmentsFromUploads() function to use the original filename, if provided and valid (and after sanitation of cause).