Martyn is away this week, so I'll see if I can help here.
I have downloaded the psdata file from your above link and will test. I can confirm I get the same error (mine appears at the top of the webpage). The file however, has been added with the option to place it inline shown below the reply box as normal.
No idea why the error is occuring, I'll discuss with Martyn next week. Let me know if you can upload and place the attachment, even with the error appearing at the top of the page.
I have tested with another file and I get the same thing... again I am able to use it in the post: