Improve documentation for handling results of uploads in express/koa
I didn't want to file an issue but am not sure where to get help.

I've followed the steps to get the express middleware working, I've followed tutorials for sending in the query from my client side, but I don't understand what to do in my express controller handling the actual file.

In my arguments, all I get is an object of `{ file: { path: 'filename.png' } }`

The file doesn't exist in the os.tmpdir, I can't use createReadStream on it, etc.

It's possible I'm not sending in the data right, but I've structured my api query just like other tutorials did:

```
query: `
    mutation($file: Upload!) {
        server:createServer(
            input: {
                bannerFile: $file,
                name: "${values.serverName}",
                address: "${values.serverAddress}",
                description: "${values.serverDescription}"
            }) { id }
        }`,
variables: {
    file: values.bannerFile // this comes directly from the files from a file input. 
}
```