Drag and drop stopped working, I think after 17.3

[zenzen]

Whenever I see this type of issue I immediately think of Flatpak. Can you confirm if this only happens between specific programs that are installed as such?

A recent example, for reference:

How to display folder Icons in colour

Just to clarify real quick what Flatpak packages are, suffice to say that they run in isolation from the rest of the system and have restricted access to the file system. Hence the unusual behavior. The Downloads folder in your user's home directory is exempted by default from this sandbox, however. If this issue is indeed due to Flatpak permissions, moving your file over to the Downloads folder, and then upload, should work. So, what browser are you using and how did you install it. If you lau…