Using Telefunc to upload files is currently only supported by encoding the file as a string, for example as Data URL. You can use
FileReader.readAsDataURL() to encode a
File object to a Data URL. However, be aware that files encoded in this way are at least 33% larger.
Another possibility is to use an endpoint separate from Telefunc to handle file uploads. For example, you can use expressjs/multer when using Node.js.