I Got the same problem Wannderer.. Post your solution if you find any.
Seems apex is the problem here.
have tried to reproduce the problem in a machine where apex is insallted. it worked fine. I've uninstalled apex from there and tried to upload the file again. it didnt work. so I have installed apex again and file upload started working again.
We do uploads (PDFs to BLOB) with ORDS 3.0 and don't use apex. Works as described in the documentation.
You have to set the mime-type of the attachment.
Here is a screenshot from a successful upload to ords with SOAPUI:
(The response is generated from our PL/SQL-upload-procedure.)
Nor sure where your form sits. We don't have one.