The named structs don't serve a lot of purpose when they are only used
in one location.

The variables map is being modified in place which is not very visible.
This can lead to unexpected bugs.

Bring the stack helper struct definition into the function
Remove the unused key field and preset the path to start with
"variables".

Instead of always doing so if the upstream request was. Only services
taking an upload need a multipart request, as we can't guarantee all
services support the mulitpart transport.

CreateFormFile sets it to application/octet-stream without the option of
specifying it, so if the upstream payload had a more specific type
that would be lost.

Multipart suggestions