There is no native support for form-data yet.
You would need to change the parameter type to String, and take the text file data out of it by using string function calls.
I think that multipart is not supported natively by Mendix because they deprecate it. I would also not recommend the use of multipart messages:
You could post this as an idea on this forum for a feature request to support multipart messages.
Thats what I assumed Joost. But changing the string is a not my prefered solution, since this functionality is of major importance. I will solve it by just going for a Binary file input of content-type: application/octet-stream.
Thanks for explanation Johan.