Add scalar_override example to File Upload

[jonfinerty]

Add an example of setting a scalar override in order to get the correct type annotation of the runtime type of the Upload scalar to the documentation, something which took me a while to work out myself. I don't know if it's worth the update, but would have saved me half an hour.

I don't think your adding RELEASE.md's for doc changes, but happy to add one if this is accepted.

Description

Short documentation about how to get file upload type annotations working based on the particular integration your using.

Types of Changes

• [ ] Core
• [ ] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [X] Documentation

Checklist

• [ ] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] I have tested the changes and verified that they work and don't break anything (as well as I can manage).