Upload form without plugin

Hello,

Actually i’m looking for made a page with a contact form with upload field for upload picture.

So for wordpress i found nothing on internet just many blog who say use plugin …
Why use plugin if we can make it ?

I found this http://trevordavis.net/blog/wordpress-jquery-contact-form-without-a-plugin/
But How modify it for put an upload field ?

Thanks for your help and sorry for my english ..