Example of form with several fields and a binary file

[vbocan]

As of now, the example shows how to handle a multipart form that has just one text field:
#[derive(MultipartForm)] struct Upload { text: Text<String>, }
Could you please update the example or create a new one showing how to handle a form that has several text fields and a binary file? This is the typical example of a user using a form to submit some information alongside a binary file.
@jacob-pro, this is valuable work, should be pulled into the official actix web multipart package.

[jacob-pro]

Completed in #9