[preview] try guessing MIME type if server can not

Makes webp/avif/jxl work on older servers with outdated mime databases.
Also makes it perform content sniffing for cases where file extensions
are totally wrong.
1 job for add-types
Status Name Job ID Coverage
failed run-test #22016

Name Stage Failure
run-test Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log