media field cck

Hi. I am trying to write an upgrade script, so that I convert my old image_attach images to media fields. For this, I need to know which code is exactly executed, when a new file gets added as media of a node.

I haven't managed to find any relevant documentation, so I am proceeding with looking directly to the code. In case you know some resource for that, it would be of real help.

Hola amigos! necesito su ayuda nuevamente...

Les comento.