As far as I’m aware, if you can see the file in your sidebar, it should be accessible from Cursor.
If you’re talking about outside of that, then no, I do not believe Cursor can access that since it won’t be included in your project index (they wouldn’t be able to access these files without indexing your entire computer’s directory).
A workaround I’ve found when wanting to deal with a specific file is opening that file in Finder/File Explorer with Cursor, then you can easily copy and paste that file into the chat window.
If you’re talking about files that are within your project repository and hence accessible directly in the sidebar, and you still can’t @ the file, then:
Check that the project is indexed in Cursor Settings > Features > Codebase Indexing.
Check your .cursorignore file to see if it’s there.