Improve file upload documentation

[MKaHead]

#2386

Merge Checklist:

• Code selfreview has been performed (Best Practices).
• Tests are sufficient.
• Documentation is up to date.
• Changelog is up to date.
• Typescript typings for properties are up to date.
• Designers approved changes or no approval needed.
• Dependencies to other components still work.

[markus-walther]

Hi Marco,

thanks a lot!
I directly edited the docs instead of line-by-line reviews, in the usual fashion :-)

Cheers, Markus