Multiple file uploads
I have not found any example of uploading multiple files, so I'm opening this issue to see if someone could help.

So, I was able to create single file upload, but when it comes to multiple files upload I've been having problems. I did this little snippet to test it, but **it only receives the first file of array**.

TypeDefs:

```js
const typeDefs = `
  scalar Upload

  type Post {
    id: ID!
    title: String!
    description: String!
    publishing_date: String!
    category: PostCategory!
    files: [String!]!
  }

  createPost(title: String!, description: String!, publishing_date: String!, category_id: ID!, files: [Upload!]!): Post
`
```

Resolver:
```js
async createPost(_, { title, description, publishing_date, category_id, files }) {

  let process_upload = async upload => {
    let { filename, mimetype, encoding, createReadStream } = await upload
    let stream = createReadStream()
    filename = slugify(filename, { lower: true })
    console.log("============ TEST INI ============")
    console.log("stream: "+ stream)
    console.log("filename: "+ filename)
    console.log("mimetype: "+ mimetype)
    console.log("encoding: "+ encoding)
    console.log("========== TEST END ==========")
    return {
      stream,
      filename,
      mimetype,
      encoding
    }
  }

  Promise.all(files.map(process_upload)).then(async function(file_uploaded) {
    const util = require('util')
    console.log("INSPECT 1: " + util.inspect(file_uploaded[0], {showHidden: false, depth: null} ))
    console.log("x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x")
    console.log("INSPECT 2: " + util.inspect(file_uploaded[1], {showHidden: false, depth: null} ))
  })

}
```