Add support to multiple files upload

[nossila]

A suggestion was made on #586 but I thought using:

type FileMap = {[key: string]: File | FileList};

was a better idea since I would not need write a loop through the FileList (since its what is provided by the file input's API) to construct an array of files every time I need to send multiple files to a Mutation, this way Relay will handle it for me, I just need to write:

class AttachMultipleDocumentsMutation extends Relay.Mutation {
  getFiles() {
    return {
      files: this.props.files,
    };
  }
}
class FilesUploader extends React.Component {
  handleSubmit() {
    Relay.Store.commitUpdate(
      new AttachMultipleDocumentsMutation({file: this.refs.fileInput.files})
    );
  }
}

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[leebyron]

I'm so sorry your PR has sat here this long. Unfortunately it does not merge well anymore and we're attempting to focus on Relay Modern going forward.

I'm going to close this PR, however if this is still important to you please rebase it and reopen