Leverage new JellyFile type in more of the codebase

[duemir]

As discussed with @jgreffe . After his introduction of Jelly Language and file type, things could be simplified.

• JellyFileTypeSchema is left but the only public API now are constants that define Jelly extensions
• Where no longer necessary checks that the current file is Jelly are removed
• Where still necessary, instead of extension, a type of file object is checked instead.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue