Add dlf file extension when saving filter

[vifactor]

If user-typed filename does not have any extension.

This small UX improvement I decided to propose after spending some time to understand why "default filter functionality" does not work even though I placed the filter file into right folder. The cause was that I saved filter in a file without dlf-suffix

[vifactor]

@alexmucde how would you want me to use the rights you granted to me? May I merge small PRs like this one which I am confident about?

[alexmucde]

@vifactor I would like to have a final look before merging, or even test the version if it is a bigger thing. But my time is limited, so i can have only a look from time to time.

[vifactor]

makes sense, of course, I just wanted to clarify