Description:
I would like to suggest a feature request for showing a prompt/warning detailing that a certain feature cannot be used with the current file type.
For example:
A user gets data output in CSV form from an external application they would like to manipulate in LibreofficeCalc. They would like to insert extra sheets, and format content with fonts and oolours and put the odd chart in here and there. None of these things can be done with a CSV filetype so it would be good to have a prompt displayed saying something along the lines of "The feature you are trying to use will not be saved to a CSV file, would you like to save this document as a xxx instead?

Currently, a warning *should* already appear when actually saving the document as CSV, but a user point can easily ignore/not read and just accept this without knowing the consequences of their actions, which will lead to loss of information.
I am proposing this warning shows up _when_ the formatting/chart/sheet is added to the document, and every time a new formatting/chat/sheet is added if the file has not been saved to a compatible format yet.

FORMATTING, FILESAVE, UI


Steps to Reproduce:
1. Open CSV file
2. Add sheets, charts, formatting etc.
3. save file


Actual Results:
If "save as xxx instead" dialog does not appear or is ignored (easily done by a negligent user), information is lost

Expected Results:
a prompt should appear when adding formatting/charts/sheets explaining those features are not available in a CSV (or TSV, etc.) file and should save as an xxx file instead.


Reproducible: Always


User Profile Reset: No

Additional Info:
This has come from a user of mine having lost a days work (in Microsoft Excel, so not for LibreOffice to worry about) because safeguards were minimal and ultimately ignored due to this exact situation (CSV file opened, features added and saved as CSV without any real checks)