Solved it for the moment by using an owncloud provided by my company. Would be good to have SFTP though to be able to use the partner company's server.

You are aware this SFTP is 27 years old now and is simply a somewhat more secure version of FTP which is largely obsolete on the internet of today. Mozilla decommissioned their FTP server several years ago and basically said the protocol was so close to dead it was not worth the effort.

There have been many filelink providers over the years https://addons.thunderbird.net/en-US/thunderbird/search/?q=filelionk&appver=&platform= if you think Kiteworks should support it, I suggest you ask them to author the addon required to implement their API.

But I am a little concerned about using public links (and that is what a filelink URL is) with restricted data. You would be much better served using SMIME or PGP encrypted email messages with the data stored on a secure server. At least with encrypted email you could use public URL's.

Mozilla decommissioned their FTP server several years ago and basically said the protocol was so close to dead it was not worth the effort.

Further information. As said Mozilla does not have the ftp:// protocol on their servers anymore (which was ftp://ftp.mozilla.org) since since August 5th, 2015 and the Firefox web browser does not view ftp:// anymore as of Fx 90.0 Release.

A very long time running popular open source FTP client FileZilla (no relation to Mozilla) supports SFTP as an option. https://filezilla-project.org/

As far as I know, Firefox only ever supported FTP, but never supported SFTP. FTP and SFTP also don't have as much similarity as the name suggests (that would apply more to the less common FTPS). Firefox having dropped FTP isn't much of an argument hence.

SFTP remains widely used, since it is typically part of the SSH server, and servers among other things as a standard method for mounting remote directories (via SSHFS).