Hi,
Steps to replicate the issue (include links if applicable):

(1) Check that you have a working dillo available. If not, install. Here, dillo is in Linux Debian 11. =8~)

(2) Retrieve a copy of the file at http://easthope.ca/OberonGadgetsIntro7.png . Only 752 bytes. =8~)

(3) start dillo, go to Commons, click on an upload button and proceed with upload of OberonGadgetsIntro7.png .

What happens?:

A screenshot of the response window is here.
http://easthope.ca/WikimediaError.png
The same result occurred in two attempts about 10 hours apart.

What should have happened instead?:

Of course the file should have transferred and the usual dialogue for auxiliary information should have appeared.

Software version (skip for WMF-hosted wikis like Wikipedia):

Occurrence in Commons a few days ago.

Other information (browser name/version, screenshots, etc.):

Package: dillo (3.0.5-7)

Test file and resulting screenshot available as noted above.

Regards, ... P. (PeterEasthope)