What’s the secret? I have used ajax uploads and interactions in Clojure forever; it’s a bread-and-butter skill to use AJAX and JSON1. However, my WordPress usage led me to believe it would be equally trivial to handle file uploads. It turned out to be less trivial than I expected, though. In summary, the secrets I needed were: Make sure the encType on the client is "multipart/form-data", which happens differently with AJAX than with plain HTML