Receiving 400 error when posting file attachment data

I'm able to create the file attachment and get a UUID back correctly. When I do the next call to upload the PDF binary data, I get an error back, 400 - No data received in POST.

I've got the file type set as .pdf in the first call and I can see that data is present in the body of the outgoing call so I'm not sure what is happening. I've tried fiddling a bunch of settings but it's a consistent error coming back.

Thanks,
Alan