Add MIME type to `get_image_metadata`

[clarfonthey]

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Are you doing the PR on the next branch?
• [x] Have you created/updated the relevant documentation page(s)?

This is required for OpenGraph under the og:image:type field, and probably other cases.

Also updated the documentation to clarify that get_image_metadata doesn't return the extension of the image, but the most common one for its format, so for example image.jpeg will show its format as jpg.