The form-data from Mendix can be used if you put correct headers.
HTTP Headers section in REST call should include all these ( No content-type header as Mendix puts it by default and even if you try to , Mendix gives you an error message)
Request section in REST call should include
This was tested with Mendix 8.17.0
In Mendix 8.9.0, an issue was fixed in posting multipart/form-data. Maybe you’re experiencing that issue?