added support for io.BufferedIOBase

[kwismann]

Added support for already open file when using send_file, there was missing a check for io.BufferedIOBase to correctly set the attribute size. This will enable support for request 'Range' headers and correct response headers with 'Accept-Ranges' and 'Content-Range'

• fixes send_file should support range request with io.BufferedReader #2566

This is my first pull request, so a bit unsure about the unchecked in below checklist.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

See issue for closure reason.