I have an acf field field where the user has uploaded a file. If i output this into a html template I see the url to the file which is exactly what I want to send to the customer on completion of the gravity form. I have created a hidden field and set the default value to be the acf field ie {custom_field:file_brochure} but instead of getting the url which is shown elsewhere, I just get the file id ie 860. I dont understand why this isnt passing through, can anyone help
Chances are, the custom field with the meta key file_brochure only contains an ID rather than a URL. ACF is probably doing some sort of work to grab the URL based on the media or post ID to get a full URL. Take a look in the postmeta table to see what the custom field with that meta key actually holds. Chances are it is just the post ID (media ID).
ACF is probably doing some work that you are not aware of and when using that ID to populate a field in your form, without that ACF function, you will have to do some work on your own.