Change default File Chooser to show supported files extensions

[morganteg]

Fixes #9372
Change default File Chooser to show supported files extensions

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Can you please add a screenshot how it looks with a single extension and with multiple?
You also need to adjust the l10n english file
https://devdocs.jabref.org/code-howtos/localization.html

[koppor]

Thank you for working on this. Easy fix :). Small comments attached.

[Siedlerchr]

Thanks for the fix! LGTM!
PS: You don't need to force push. When we merge, we have the option to squash all commits into one

[koppor]

Moreover, seeing the new commits eases the second review / check if the comments are adressed