I am new to Joplin (and note taking apps in general) and have just played around with this as well.
For me, on a mac, linking to local folders works with ‘file://’ in front of the link, as long as the link does not contain white spaces. Clicking the link then opens the folder in Finder.
Linking local files does not work for me.
Generally, should linking local files work? And, if not, how difficult would it be to include the functionality? (Something like right click: open with?)
For me, as a newbie to note taking, this seems like the most important feature of all, because I work as scientist, and code and plot and write latex notes, and now I want to link to these folders/files from within my notes. Importing everything would not work or make sense…
How do other people deal with issues like these?