When saving a text document I chose a file name like "article.first" and wrote it into the field. The file was saved without the extension *.odt as is.
When saving first time or with 'Save As' there is an option to 'Automatic filename extension', marking it should be enough.
what a dumb excuse
Such option it's enable by default, so maybe the mistake was on you or someone using the program who change it. BTW, how bad wasting my spare time with such an ungrateful person.