Feature/restrict image types

[guidotrueb]

Semantic versioning

This PR is a Major (breaking change, existing implementations will fail)

Generall things to consider

• I have read the CONTRIBUTING.md document.
• If my change requires documentation, I added it already.
• I have added tests to cover all my changes.
• All new and existing tests pass.

Notable changes

Added

• Restriction on image MIME types on both image (pictures) and logo in attachments.

Reasoning behind the changes

• If we are going to add a new restriction at all, I believe it should be as restrictive as possible, since adding more supported types doesn't break compatibility.
• We've considered market share (https://w3techs.com/technologies/overview/image_format/all) and technical limitations/recommendations regarding pictures (of course it should be discussed and agreed, especially regarding logo).
• Instead of creating a new type, reusing mimeType and adding enumeration inside the element makes it possible to have different image types for logos and pictures and still follows existing conventions.

Disclaimer: I work for homegate.ch