I confess I’m not familiar with ownCloud but its successor uses a very near identical desktop application. Nextcloud only supports partial selective sync and does so by folders, not arbitrary file extensions. There’s an option to specify a list of filetype exclusions but not a list of inclusions; you could manually enter all the filetypes you wished to exclude (.doc, .docx, .rtf, .pdf, .jpg, .jpeg) but that’d be very time consuming and some would slip through the net.
You could look for some other software because WebDAV offers you the flexibility to choose.
You can’t only upload certain filetypes (.axx) in the GUI. You can on the command line but that’s not a good fit for Windows 10 unless you get the Ubuntu bash shell – Windows Subsystem for Linux (Beta).
I think you’d have to create various Secured Folders in AxCrypt to achieve what you want and run a manual sync in ownCloud once you’re satisfied that everything is encrypted.
AxCrypt don’t allow you to get an offline license file without sending your password. You’d have to connect to the internet, purchase it and copy the file to whatever system you’re using but you’d still have sent the password to AxCrypt. A few people have requested this if you look through the forum. You must submit your password to the server and you must trust that they retain it only in RAM. You could use the always offline mode [-offline] and make an account but you get AES 128. If you want StrongerEncryption you need to obtain the source code and modify it. It’s a very simple thing for them to add but it probably comes down to a lack of resources.