I think this is now fixed.
Following this thread: https://processwire.com/talk/topic/1002-cant-upload-imagefiles-problem/
I created /site/assest/uploads which didn't previously exist and then added this line to /site/config.php immediately below the debug line
$config->uploadTmpDir = dirname(__FILE__) . '/assets/uploads/';
It seems the temp upload directory was either not set or not writable.
Hope this helps someone.
Thanks Tom. Although I had tried that idea your message prompted me to dig a little deeper. It does work but...
The category I was testing was titled 'Hints & Tips'. Even though both the page title and category title were cleaned so the '&' was presented as the HTML entity '&' in both cases the selector still doesn't work. However, changing the title of the category to 'Hints and Tips' has made the selector you shared work.