custom file upload

hi, i need to validate if a file is uploaded in the registration form. This has not to be a required field but once you are login it has to validate if the file is uploaded. what hoook should i use?

i was thinking in

add_action(um_user_login, my_function)

function my_function(){
file_exists(“my_file.pdf”)
}

but i dont know what to put in the string arg in the function. Where supposed to be located the files that are uploaded?