Can't properly drag and drop icons from the desktop to the browser

Steps to Reproduce The Issue

1. open a Chromium-based browser and enter https://postimages.org
2. have an image on the desktop
3. try to drag and drop this image to the browser

Expected Behavior

The website should detect the file and upload it. This issue is not limited to this website (confirmed) and probably not limited to Chromium-based browsers either (haven't tested).

Current Behavior

The image file is downloaded into the Downloads local folder, instead of being uploaded.

Environment & Versions

Tested using Xfce 4.19 built from master today (xfdesktop 8bdd17d3fc6b607b4eeb3cd6db5f074421719005) on PorteuX 1.8 current + X11.

Reverting Xfce to 4.18.x fixes the issue.