Renaming a file should re-add the extension if missing

[Siedlerchr]

Is your suggestion for improvement related to a problem? Please describe.
When I rename a file, the extension is not kept

Describe the solution you'd like

1. I add a file to an entry. I want to rename the file For that, I press Ctrl+R
2. The rename dialog appears - the whole filename is selected:
   
3. I can type a new file name (e.g., demo
4. I press Enter

• Expected result: The file is named demo.pdf
• Actual result: The file is named demo (without extension)

JabRef should check whether the user removed the extension. If JabRef cannot find a new extension, it should put the old one (and not just keep the file as is)

Other implementation possibility: The dialog should select the filename only - without extension.

[ivobubenko]

Hi, I would like to work on this issue, please assign it to me.

[github-actions]

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