Challenge 2: Saving Edits to a Colleague's .xlsx File — Solution Walkthrough

Expected dialog:

A prompt appears asking whether to keep the file in its current
Excel format ("Use Excel 2007-365!" or similarly worded) or switch it
to LibreOffice's own native ODF format ("Use ODF Format!").

What the two choices mean:

  - "Use Excel [format]!" — saves the file back out as a genuine
    .xlsx file, in Microsoft's own format, exactly as it was received.
  - "Use ODF Format!" — converts the file to .ods, LibreOffice's own
    native Calc format, changing its actual file type in the process.

Which one to pick if the file needs to go back to that colleague:

"Use Excel [format]!" (keep .xlsx) — since the colleague uses
Microsoft Office, they need the file to remain in the format their
own software natively expects. Choosing "Use ODF Format!" here would
convert the file to .ods, which may not open cleanly in their copy of
Excel at all, or could arrive in a format they don't recognize as the
file they originally sent.

WHY THIS WORKS AS AN ANSWER
------------------------------
This is a direct, concrete instance of this chapter's own warning box:
the save-format prompt exists precisely because a Microsoft-format
file being edited in LibreOffice needs an explicit decision made about
which format to save back out in — clicking through it without
reading which button does what risks silently converting a file the
recipient specifically needs to stay in its original format.