Simple file input example

There are no examples in the documentation about file inputs, and the issues advise to use react-dropzone, which is overkill if I just want a simple file input.

So I just wanted to share a TypeScript snippet that can be used for file inputs, especially if you want to have a `File` in your field value (instead of that useless _fakepath_ given by the input `value`), if you want to create a `FormData` for example.

```tsx
import type { InputHTMLAttributes } from "react"
import { Field } from "react-final-form"

interface Props extends InputHTMLAttributes {
name: string
}

const FileField = ({ name, ...props }: Props) => (
name={name}>
{({ input: { value, onChange, ...input } }) => (
onChange(target.files)} // instead of the default target.value
{...props}
/>
)}

)
```

The trick is to just replace the default `change` event behavior by giving it the `files` attribute instead of the `value`, and leave the input uncontrolled since we extract and ignore the `value` ([this is normal for file inputs](https://reactjs.org/docs/uncontrolled-components.html#the-file-input-tag)).

Then in your `onSubmit` handler, you will have a `FileList` as the field `value`, so you can iterate over it (or just read the first element if it's not a `multiple` file input) and add each `File` to your `FormData`, e.g. if you form has a ``:

```tsx
const handleSubmit = async (values: Values) => {
const payload = new FormData()
payload.append("file", values.files[0])
// you can also add the rest of your form text data, e.g.:
payload.append("firstname", "Antoine")
payload.append("lastname", values.lastname)
// and then just post it to your API using a regular POST:
await fetch(YOUR_URI, {
method: "POST",
body: payload, // this sets the `Content-Type` header to `multipart/form-data`
})
}
```

See it in action: https://codesandbox.io/s/react-final-form-file-field-ffugr

Related issues:
#24
#92

If you like this example, I can make a PR to add it to the docs.
Or it could be the default behavior of the lib for file inputs.