Drupal 8’s REST API has been maturing steadily since the Drupal 8.0.0 was released in November 2015. One of the big missing features has been file upload support. As of April 3 2018, Drupal 8.6 will support it, when it ships in September 2018! See the change record for the practical consequences: https://www.drupal.org/node/2941420.
It doesn’t make sense for me to repeat what is already written in that change record: that already has both a tl;dr
and a practical example.
What I’m going to do instead, is give you a high-level overview of what it took to get to this point: why it took so long, which considerations went into it, why this particular approach was chosen. You could read the entire issue (#1927648), but … it’s one of the longest issues in Drupal history, at 572 comments1. You would probably need at least an entire workday to read it all! It’s also one of the longest commit messages ever, thanks to the many, many people who shaped it over the years: