POST to /texts/ endpoint should accept .tess file separate from JSON metadata

[nOkuda]

Currently, POST to /texts/ endpoint takes the .tess file as a string in a JSON object as part of the request body. This is not standard practice, as @jeffkinnison notes in #32.

It remains unclear to me what standard practice is. https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST notes that enctype=multipart/form-data is standard when working with HTML forms. But what should be done when sending a POST request that is not through HTML forms?