Thank you Geoorg! I have done it already, that is checked the "Clear default" in all my 3 installed mail apps. And there were no defaults set. But you mail induced in me to second thoughts and so I checked also the receiving app - that is "File manager". There were some defaults set and when I...
When using the Fair mail app and trying to view I once by mistake tipped "Remember my choice" and "Save" (at least I think so) and from that moment the phone allows only to save the file, not to first view it. This his happening only with attached MP4 files, the same story repeating itself also...