Umbraco is an ASP.NET content management system (CMS). Starting in version 14.0.0 and prior to versions 15.4.2 and 16.0.0, it's possible to upload a file that doesn't adhere with the configured allowable file extensions via a manipulated API request. The issue is patched in versions 15.4.2 and 16.0.0. No known workarounds are available.
https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-fr6r-p8hv-x3c4
https://github.com/umbraco/Umbraco-CMS/commit/d920e93d1ee29dc3301697e444f53e8cd5db3cf9