[Bug] Modlist export unimportable

[echoAwoo]

Background

• Operating System: Windows 10
• CKAN Version: 1.31.2
• KSP Version: 1.12.5.3190

Have you made any manual changes to your GameData folder (i.e., not via CKAN)?
No

Problem

Describe the bug
Saving mod list only supports the following file types:

Modlist import only accepts CKAN filetype.
Manual reassignment of any export file types is not compatible with the expected JSON format the importer uses.

UI degreys and then locks up after error message is closed requiring taskkill to close application

[HebaruSan]

Use File → Export modpack... to generate .ckan files:

• https://github.com/KSP-CKAN/CKAN/wiki/Sharing-a-modlist-%28metapackages%29

[HebaruSan]

(Related to #3771)