good question. I don’t think that hook will work because you can’t change the file name, only add errors for custom validation.
I would use the wp_handle_upload_prefilter hook.
Take a look in the file …/plugins/advanced-custom-fields-pro/includes/api/api-helpers.php
There is a function in there named get_source_field() that shows how ACF gets the field associated with the upload. I would do the same thing to determine if the upload is for an ACF field, what field, and then alter the file name if it’s the right field.
