Hi Tim99!
I'm having some issues with other things since I upgraded to 2.4.0-rc, but am able to upload. The funny thing is that I lost the ability to upload after a previous upgrade. I tested a few different file types (jpg, doc, pdf) and had no problem. My best suggestion is to use firebug to see if you can find out what is causing the error.
I know it makes the concept of a cms seem ridiculous, but you should be able to upload directly to the assets folder on your server then access the files from the cms. When my upload wasn't working I had to upload directly to the server, refresh the cms then click on the files & images tab before I could see the files in the upload section of the interface. It didn't matter if I actually saw the file or even open the folder it was in, I just had to go to the files & images tab.
Sorry I can't be more helpful,
HB