Hello,
Just found this from your old thread. It seems we aren't communicating properly and maybe I'm missing an important detail to your situation. A Fresh install of Spark 2.6.3 as downloaded from this site should already have File Transfer built in as a base feature of Spark. There is no plugin needed to send files to other Spark users from your computer. I cannot speak for any other XMPP Client program (including Pidgin) as I only use and work on Spark at the moment. Perhaps you are trying to send a file to a user with a client program that is not capable with File Transfer?
To do a File Transfer from 1 Spark user to another Spark user on the same XMPP server:
1) Double click the user's name from your roster list or open a new Chat.
2) Click on the image icon in the top left of the window that is the picture of the hard drive with the green arrow pointing upwards on it
3) Select the file to send
4) Wait for other user to accept incoming File Transfer
5) Wait for File Transfer to complete
I've attached a screenshot showing a normal Chat window. I drew a red box around the File Transfer button. I blacked out some of the spots that are non-standard as not-to-confuse anyone.
Hope this helps!