guess the content_type in local_file cameras

[azogue]

Description:

Guess the mime type of the local_file camera to set it correctly for each file, instead of using always the image/jpeg type.
This change allows to include local SVG files as Home Assistant cameras, and it's the analogous change to that produced for the generic cameras in PR #8188.

Example entry for configuration.yaml (if applicable):

camera:
  - platform: local_file
    file_path: /path/to/file.svg
    name: SVG Camera

  - platform: local_file
    file_path: /path/to/file.png
    name: PNG Camera

Checklist:

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.