refactor: changed the way messages holds files

[Jaskowicz1]

This PR changes the way that messages hold files. Rather than there being 3 vectors of strings for: name, content and mimetypes, there is now a vector of message_file_data structs.

This doesn't break the way you add files, but rather breaks the way you read files you've added to a message.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[Jaskowicz1]

This is proof of it still working!

[Jaskowicz1]

Reverted to draft as more changes are required.

[Jaskowicz1]

No longer in draft, ready for review!