Right now there is no built-in option for importing into file fields from the builder.
If you’re familiar with APIs, then you could use the Knack API to import files: Knack API - File/Image Uploads.
Live App Import
If you want your users to be able to import multiple files into a single file field, you'll need to change your app structure a bit. For example, you may want potential tenants to submit multiple documents when creating a single tenant application.
A file field can only store a single file, so you'll need a connected object to store the multiple files for a single record. This article shows you how to set that up: Store Multiple Images or Files for Individual Records.