Ejabberd: 'mod_http_upload' is included in ejabberd-core since version 15.10.
After you have connected to a server with XEP-0363 support, you should see a new button for file transfers.
When the upload is completed, a thumbnail will be sent to the recipients with XHTML-IM. If the recipient doesn't support XHTML-IM, they will just see the URL instead. Gajim supports XHTML-IM and will always display the thumbnail.