aspupload - File(s) not uploaded

This post is more for my reference as I still work on a number of classic ASP projects which use aspupload in my day job. When attempting to upload images via an upload form in the CMS a client was getting the error:

File(s) not uploaded.

After 2hrs of trial and error and attempting various fixes including IIS security settings and folder permissions, I was finally able to narrow it down to the form tag. I had omitted: