ACF stores both Image and File fields as the ID of the media attachment.
In order to put the URL into a field you’ll need to create another field to hold the URL, like a text field. Then you’ll need to create an acf/save_post filter, this is outlined here: http://www.advancedcustomfields.com/resources/acfsave_post/.
You’re filter will need to retrieve the ID value and convert it to a URL and then update the text field.
