Insecure File Upload in the indexing/preview REST endpoint

In Splunk Enterprise versions below 9.2.2, 9.1.5, and 9.0.10 and Splunk Cloud Platform versions below 9.1.2312.200, an authenticated, low-privileged user who does not hold the admin or power Splunk roles could upload a file with an arbitrary extension using the indexing/preview REST endpoint.