Been using a handful of images in my sandbox for a while now, but all of a sudden the images have gone, replaced by empty image holder

I am getting

Error
Mime type not valid (), allowed Mimes are image/gif - image/jpeg - image/png :

Impossible to upload file : filname.png

Is this just a safety configuration you have put on your sandbox to stop uploads (and save space) or a problem that will propagate down to live installs.

Fixed