Firefox plugin download full file and format it's name

I started using the firefox plugin recently as I finally got JabRef 5 running on my linux machine.
What I noticed was that instead of downloading the full pdf, it just sets a link. Is there an option to download the full file should one be available?
I now resorted to adding with the plugin and then looking for the fulltext pdf from JabRef. However, I would like to have the filename formatted in a very special way. Seems like this was possible with earlier versions, as described here, but those strings do not work with JabRef 5. Is there any documentation on how the strings should look?
This formatter:
for example results in the bibtexkey (as wanted) and then
I just tried that string as it was supposedly working at some point, I actually would like to have the bibtexkey followed by an underscore and then the title, with all non-letter characters replaced by an underscore and all whitespaces replaced by a dot. Is that possible?

Aha, formatting was changed to how bibtexkeys are formatted (see here)
However, I still have to find out how replace works here. I still don’t understand how the regex replace works here.

This is my format string now:
[bibtexkey]_[title:regex(":","_"):regex("/","_"):regex("-","_"):regex(" ",".")]
It does not catch all non-alphanumerical characters as I wanted, but using \W crashes the application and so does trying to escape the dot for example. I will stick with this one as it will cover most cases.
However, it would still be nice to get the plugin to automatically download pdf files…

Looks like automatic download is not supported yet, but someone is working on it.

I started to work on the issue myself and drafted an implementation. Check it out here!