Allow single file uploads

[jaydenseric]

Basically, allow for mutations instead of just like this:

updateAvatar(userId: String!, file: [UploadedFile!]!): Avatar

Also like this:

updateAvatar(userId: String!, file: UploadedFile!): Avatar

• Clearer semantics.
• Better validation for mutations requiring only one file in a variable.
• It is sometimes easier to get a single File object to send as a mutation parameter:

handleSubmit = event => {
    event.preventDefault()
    const formData = new window.FormData(event.target)
    this.props.mutate({
      variables: {
        userId: this.props.userId,
        file: formData.get('avatar') // Instance of File (not FileList), from <input type='file' name='avatar' required />
      }
    }).then(({data}) => {
      console.log('Avatar updated', data)
    })
  }

To get this to work, handle File as well as FileList here and here.

[jaydenseric]

Not certain how all this works, but here in graphql-server-express-upload also looks relevant.

[jaydenseric]

For those who find this pressing, I ended up supporting single File objects, File arrays, and FileList objects anywhere within mutation or query variables with apollo-upload-client.