Action "Copy to the clipboard" does nothing

To reproduce:

1. Start xfce4-screenshooter
2. Make a screenshot
3. Select action "Copy to the clipboard"
4. Click Ok
5. Try to paste the screenshot in any program (eg GIMP, or Discord/Matrix in a browser).

Expected result:

An image gets pasted.

Actual result:

Nothing happens (or in the case of GIMP, a message saying that the clipboard oes not contain image data is displayed).

Workaround:

Choose the option "Open in GIMP" and copy it from there.

Downstream report:

https://bugs.launchpad.net/ubuntu/+source/xfce4-screenshooter/+bug/2115460

Identical behavior was verified with f2f8774c - git master at time of writing.