digiKam is a software originally programmed for linux, now also available for windows ( https://www.digikam.org/ ). It’s an excellent free photo management software and has several exports integrated: to dropbox, flickr… but one is missing: seafile.
You’d probably be needing to ask on a digikam forum for this.
However, assuming you don’t want to simply add your digikam folder(s) as a Seafile library for syncing, you can still actually drag and drop photos from digikam into the seafile client’s “cloud file browser” to easily upload photos to a seafile server for which you don’t keep a local shared library. It even works with dragging folders from digikam to upload whole abums at a time (i just tried it to make sure). Drag and drop onto the Seafile web interface also works to upload folders from digikam (at least it does for me using Digikam 5.2, Chrome and Vivaldi browser, with KDE desktop on Arch Linux).
That sounds like a great way to do the task I wanted.
But I get an error if I drag a folder from digiKam into the seafile-file browser: The german error message (see the screen shot) translates to something like “function not supported” (windows 10, seafile client 6.0.0, digiKam 5.2). I get the same error message when I drag and drop a folder from the windows file explorer.
Anyhow uploading selected files works - also a functionality I did not know about before. Thank for that hint!
EDIT: I use different seafile servers. Using the seafile server 6.0.5, I get the error message. Using seafile server 5.0.4 dragging-an-dropping a folder works (using the same windows client 6.0.0)
My server version is 6.0.1 pro, and the drag and drop from digikam works fine (client version 6.0.0). I can’t imagine what the difference between your 5.0.4 and 6.0.5 server could be, but I would think it should work the same. Have you tried drag and drop into the seafile web browser interface to see if it behaves any different?