Can I set part header manually?

multipart_data:set_simple is useful, but I need a new method to upload file with header like:
```html
--------------------------bd99c856fd831904
Content-Disposition: form-data; name="record"; filename="run_test.sh"
Content-Type: application/octet-stream

../../bin/lua run_test.lua $@

--------------------------bd99c856fd831904--
```

Any suggestions? 