If I understand you correctly, you want to compose a multipart request manually from an HTTP/REST console. The multipart format is simple; a brief introduction can be found in the HTML 4.01 spec. You need to come up with a boundary, which is a string not found in the content, let’s say HereGoes. You set request header Content-Type: multipart/form-data; boundary=HereGoes. Then this should be a valid request body:
--HereGoes
Content-Disposition: form-data; name="myJsonString"
Content-Type: application/json
{"foo": "bar"}
--HereGoes
Content-Disposition: form-data; name="photo"
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
<...JPEG content in base64...>
--HereGoes--