For the batch import, you need to select a document directory in Settings > General Settings > Document Directory. Then the app will look in that directory for files to import.
For the single file import, Android should let you select any plain text files, regardless of file extension, but sometimes it gets confused between plain text and binary files and shows some plain text files as not selectable. If that's happening, you can turn on Help > Utilities > Use Old File Picker and then it will look at the file extensions, but then you will only be able to select local files, not files on cloud storage.