Basically, I am using the one from Valeriy Kholodkov: http://www.grid.net.ru/nginx/upload.ru.html
upload_add_header feature appeared in v 2.2, I guess:
https://github.com/vkholodkov/nginx-upload-module/blob/2.2/ngx_http_upload_module.c
I can find it in C code - 'upload_add_header'.