Blood for the Baron!

Attachment uploading doesn't seem to be working today. Getting an "http error" message when trying to attach an image to a message. 2.73mb image file using Firefox 44.0.2

Seems to be working... what file type is it?

Still having the same problem here. It's a jpeg image. I've tried a smaller image and that worked so I guess it's a file size thing.

Scratching my head, I'm afraid, everything is set to unlimited.

It's probably limited to 2mb in your PHP config, that's usually the default setting. If you have access to your .htaccess file you might be able to overide it in there.