Create pinned file request body

------WebKitFormBoundaryJ0NuQRUCiq8VBMTw
Content-Disposition: form-data; name="content"; filename="IMG_1610.JPG"
Content-Type: image/jpeg

{FILE_CONTENT}
------WebKitFormBoundaryJ0NuQRUCiq8VBMTw
Content-Disposition: form-data; name="file_info"

{"position" : 15}
------WebKitFormBoundaryJ0NuQRUCiq8VBMTw--