Writer provides the option send under the file menu. There one can select: send document as microsoft word file in an email. Here a .doc file is generated instead of the now common .docx . The .doc format is falling into disuse, and might be supported by less programs than the ubiquitous docx file format.
It's the same for Calc and Impress> xls/xlsx, ppt/pptx
Setting as new enhancement - make it configurable.
Samuel Mehrbrodt committed a patch related to this issue.
It has been pushed to "master":
tdf#99122 Send as email: prefer ooxml formats
It will be available in 6.2.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
*** Bug 86920 has been marked as a duplicate of this bug. ***