Good morning Guru Community! 
We recently shipped a small update to individual Card sharing settings. Now, when public Card sharing is disabled for a Collection, the checkbox for the ability to share attached file links publicly (when creating a Card in that Collection) is also disabled.
Existing published Cards file sharing permission settings can be updated in bulk using Card Manager. Learn more in the Help Center article Changing a Card's Privacy and Sharing settings.
Reach out to a workspace admin if you’d like public Card and file sharing to be enabled for a Collection you’re working in.