There's no easy out-of-the-box way yet. But you can do it today with JS interop as a workaround. For small files: read the file and send the bytes directly in a Hologram command payload. For big files: fetch-POST the file to your own Phoenix endpoint, or directly to S3 with a presigned URL (no extra endpoint needed, since presigning can happen in a regular Hologram command or in the page's initial state), then pass the returned reference to a command for processing.
A proper Hologram-way file upload is planned as part of a high-level forms abstraction, which comes after Local-First support lands.
I've tried to keep an eye on its development, but I don't think file uploads were mentioned yet (and there's nothing about them in the docs).