I believe there should be two FILESAVE codepaths, a 'pure' one for adhering to the specs and writing sexy XML, and another for emitting more 'MS Office compatable' (yet still valid) OpenDocument files.
For example (bug #53997), MS Excel does not understand
and instead requires
Both are valid code. Both can be read by LO and other free OpenDocument-supporting apps, yet only the latter works in Excel.
The default should be MS-compatable saved files. By saving files that MS Office can read without error (whilst still valid, just by using alternative XML) LO will be able to integrate more easily into existing Microsoft-dominated environments.
No more cries of "This '.ods' thing you sent me is corrupt, just send me an Excel file!" due to bugs/inadiquecies in MS's OpenDocument support.
CCing Calc expert from https://wiki.documentfoundation.org/FindTheExpert.
Please comment and set status accordingly.
*** This bug has been marked as a duplicate of bug 53997 ***