The responses you got I think literally answer your question but probably aren't what you're going to reach for any kind of HTTP based thing. Your go-to should

The responses you got I think literally answer your question but probably aren't what you're going to reach for any kind of HTTP based thing. Your go-to should probably be multipart/form-data. It's well supported in every language and HTTP library and you can send both JSON and the file in the same payload. There seems to be a common trend of people writing "JSON APIs" thinking that every other part of HTTP is off-limits.