This capability did not disappear; it was never present. You can only attach files to transaction forms, not reports, because the report is not maintained as a record. Only the parameters that define it are stored. Reports are actually recreated every time they are viewed.
You don’t need to add your custom field from scratch every time. You can set it up under Form Defaults so every new customer gets it, prefilled, every time. This will affect every new customer (but not pre-existing ones).
By the way, @Jon is not a developer. He is/was a user, but has not been active on the forum in almost two years.
please be advised that no support will be provided related to the theme in case it breaks due to an update to the software. also, for modifications and updations you will have to hire a local programmer if you do not have the necessary expertise yourself.