So, recognizing this data:image part as inline image data, I decided to try a trick and copy the link except remove the URL to my board, and try to navigate to it with my browser. I saw the image render in the tab, saved it to my computer, and edited the original post to remove the broken image and uploaded an attachment in its place. It was 274kb. Somewhat related, I think, is that my Chrome cache became corrupt after viewing this page and trying to load this "image" (every page load after that results in a long wait time "Waiting for cache..." before anything starts to load - had to delete the cache folder)
What happened, I believe, is that the poster dragged an HTML inline image from somewhere to his post. The board (or the editor) processed it as a URL with the base being the board URL. I have reproduced this here, with a small image so as not to cause a 414 error/cache breaking:
Would it be possible to have the board convert this to a file attachment automatically?