FileStorage perhaps has incorrect typing metadata

Describe how to replicate the bug.

    from flask import request
    # snip some app-specific logic...
    fileitem = request.files["file"]
    blob = fileitem.read()

and then:

    mypy test.py
    test.py:3: error: "FileStorage" has no attribute "read"

Environment:

- Python version: 3.9.5
- Flask version: 2.0.1
