.nupkg is not a supported extension but .nupkg may contains files which have extensions supported by CAST Highlight for SCA, for instance .dll.
Solution
If you want to retrieve the component corresponding to a .nupkg, you can use the recent Component Catalog Management feature where you can submit your file to display component's meta data (license, possible vulnerabilities, etc.).
Additional Resources
CAST Highlight Troubleshooting Guides
CAST Highlight Product Documentation
Ticket Number
24621
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