Better support for multipart/form-data file upload examples

Currently in the JS / Fetch example, it shows multipart file uploads like so:


This doesn't work as it just sends the string "logo.png" instead of the file.

A better example might be:

```js
// HTML file input, chosen by user
const fileInputs = document.querySelectorAll('input[type="file"]')

form.append("file", fileInputs[0].files[0]);
```

Start by locating the JS/Fetch multipart/form-data example and the code that generates its file-upload snippet. Compare the current output with the issue's File-based example, then verify that generated multipart examples reference the selected file rather than the string "logo.png".

索引モデルが issue の本文から書いたものです。