I get your problem, you add a link type attachement and it should let you upload the resource to download but it only accept img types, so you can not upload a PDF. I think on webstudio we can upload images only (I can be wrong )
You can link a file and make it downloadable, but you have to host it somewhere else, e.g. dropbox/googledrive etc. We currently don't support these type of assets
Figured out the reason, somehow we lost the input field there, which allows you to link an external resource and we automatically would make it downloadable