The key step was setting enctype="multipart/form-data" on the <form> element. If you skip that step, you'll only get the file's name on the server.

Currently (as of 4/14/24) this only works for small files (< 1mb). For larger files you get an error: {"statusCode":413,"error":"Payload Too Large","message":"request entity too large"}.

A workaround is to upload to another service client-side, send the URL from that service to your Val Town server, and then fetch the file server-side (helper: @stevekrouse/uploadTo0x0).