If you have the file ID you can either do a query of posts using WP_Query for posts that belong to the user and a meta_query to look for the attachment id in in the field where it is added.
This is only an example, you’ll need to alter to your values and test
$user_id = get_current_user_id();
$args = array(
'post_type' => 'the-post-type',
'author' => $user_id,
'posts_per_page' => 1, // only need to see if one exists
'meta_query' => array(
array(
'key' => 'custom_field_name',
'value' => $file_id
)
)
);
$query = new WP_Query($args);
if (count($query->posts)) {
// this file is attached to at least one post by this user
}
