How to get profile user id when uploading image via media uploader on profile page

I have a PlugIn which lets a user upload an image to a user profile in the backend. Now I want to access the user id in the uploader to change the filename of the uploaded image.

On the user profile edit page, I get the id via global $profileuser. But when I access it in a function I added as a filter to wp_handle_upload, $profileuser is empty.

Any ideas how to get the profile user (not the logged in user) in this case?

This is my code:

add_filter('wp_handle_upload_prefilter', 'my_pre_upload', 2);

function my_pre_upload($file){
    // get current user
    global $profileuser;
    $myAuthorImg = get_userdata( $profileuser->ID );
    $myAuthorImg = 'author-' . $myAuthorImg . '.jpg";
    $file['name'] = $myAuthorImg 
    return $file;
}

You need to pass the $profileuser->ID as the second argument when you call your function and define it accordingly:

add_filter('wp_handle_upload_prefilter', 'my_pre_upload', 2, 2);
// (3rd param is priority, 4th is number of args)
// and pass the $userid as argument to your function

function my_pre_upload($file, $userid = false){
    // if no user specified, get $current_user
    $userid = $userid ?: get_current_user_id();

    $user = get_userdata( $userid );
    $myAuthorImg = 'author-' . $user->ID . '.jpg";
    $file['name'] = $myAuthorImg 

    return $file;
}