For Kentico 13 you can use REST API and request the bizformitem data as described in the documentation.
In Xperience by Kentico REST is not available, but you could use GraphQL - but this is designed to be used with headless channel.
In Kentico 13, the form attachments are stored on disk. So, the actual value in the attachment field is the GUID of the file - so, this GUID value is returned in the JSON. I would recommend using PostMan and do some tests to see how it works and looks like.