Addendum:-

The root cause of this problem is by default Opera browser is saving all files in xlsm format because of the tick mark made in Remember The Choice while downloading the file.

So in opera browser memory it made all unknown files (according to opera browser xlsm/xlsx/xlsb are unknown file types) so whatever the file which is not in opera library will be treated as unknown file type and due to the option made above (Remember The Choice) opera keep on saving all the unknown file types as .xlsm which caused this problem.

To remove this from opera memory:-
Settings>>Preferences>>Advanced>>Download>>Select Unknown (Nearby You can see the default file type(extension) assigned by Opera for saving Unknown File types)>>Delete>>Ok

Problem solved