Encoding of filename for MIME attachments

I'm using a REST service to upload files using MIME attachments.

It should work with Swedish characters but the web service expects the filename in the header to be encoded as utf-8 for non us-ascii characters to work.

Inspected the request from both SoapUI and Postman using Fiddler and Postman correctly sets it utf-8 automatically.

However, SoapUI does not and I can't figure out to set this. Tried changing the default file encoding in vmoptions, setting the ContentID according to RFC 2047 but without success.

When using RFC 2047 encoding actually only "name" gets set but not filename.

Any ideas?