Not sure of the right way to do this, but in the past we have used a custom restlet that accepted a JSON document with the file data being base64 encoded into one of the JSON properties.

i.e.
the client would send something like this:

{
“fileName”:”myfile.txt”,
“fileData”:”aGVsbG8gd29ybGQ=” /* hello world */
}

and then we would have a restlet to decode the base64 “fileData” property and store it into a file.  Something like the following:

define(["N/file", "N/encode"], function (file, encode) {
    function post(requestData) {
        const fileId = file.create({
            name: requestData.fileName,
            encoding: file.Encoding.UTF_8,
            contents: encode.convert({
                inputEncoding: encode.Encoding.BASE_64,
                outputEncoding: encode.Encoding.UTF_8,
                string: requestData.fileData,
            }),
            fileType: file.Type.PLAINTEXT,
            folder: 12 /* replace with your folder to store into */,
        })
        return {
            "fileId": fileId
        }
    }
    return {
        post: post,
    }
})