Fix missing .json file extension when exporting messages

[tom93]

Very similar to FossifyOrg/Notes#14 (which has a more detailed commit message). Basically, some file pickers don't automatically append the extension, so we should do it ourselves. It doesn't cause problems.

What is it?

• [x] Bugfix
• [ ] Feature
• [ ] Codebase improvement

Before/After Screenshots/Screen Record

None, but see FossifyOrg/Notes#13 for screenshots of a similar issue in the Notes app.

Fixes the following issue(s)

• Fixes part of #88

Acknowledgement

• [x] I read the contribution guidelines.

[tom93]

Should I also port this to the Phone app?