Double-clicking on a .desktop file opens it in Mousepad instead of running it

I don't know if this is the desired behavior now (it seems to me that there have been security related changes in this regard), but in any case the behavior has changed between Thunar 4.16.11 and 4.17.12, and I find that this could appear as a regression.

Steps to reproduce:

• Download Tor Browser.
• Extract the archive.
• Double click on the .desktop file in the extracted directory.

Thunar 4.16.11: Tor Browser starts.

Thunar 4.17.12: The file opens in Mousepad or other default text editor.