I was going to reply this:
Instead of selecting ‘Form data’ as type of the request, you could select ‘Custom request template’ and build your request text-based yourself.
However, if I see your examples, I think you would like to have a binary multipart message, something that wasn't possible at all in Mendix a while back. So, while I'm surprised about the new possibilities in Mendix, I guess you're disappointed that the key of the form part isn't dynamic.
Depending on the possibilities of the receiving end, maybe you could consider workarounds?
Otherwise, you are indeed dependent on custom Java actions to do something like this. I don't see existing solutions providing for this issue in the marketplace.