Hello. When uploading a simple 4kb SVG on a freshly installed private test wiki running MW 1.35, I get the above error. The only modification done to the installation was adding SVG to $wgFileExtensions.
After uploading the SVG file, the file page displays the SVG. It just doesn't display in the "Thumbnail" cell in the "File history" section, and does not display on any other page when used. The above error is displayed in its place instead.
On the hosting platform/environment running 1.34, the same works perfectly fine.
To be 100% sure, I had also retried installing fresh on another subdomain. Same result.
Any idea what has changed/how to solve this issue?