If you run ZAP with the "-h" option you'll get some help on the commands available:
./zap.sh -h
Found Java version 1.7.0_71
Available memory: 16003 MB
Setting jvm heap size: -Xmx512m
GUI usage:
zap.sh [-dir directory]
Command line usage:
zap.sh [-h |-help] [-newsession session_file_path | -session existing_session_file_path]
[options] [-dir directory] [-installdir directory] [-host host] [-port port]
[-daemon] [-cmd] [-version]options:
-script [script_path]: Script to run from commandline or load in GUI
-quickurl [target url]: The URL to attack, eg
http://www.example.com -quickout [output filename]: The file to write the XML results to
-last_scan_report [file_path]: Generate 'Last Scan Report' into the file_path provided.
You'll see that the -quickout option only generates XML results, regardless of the extension of the filename you specify.
What OS are you using?
You could try applying the XSLT file that ZAP uses to transform the XML results to HTML:
https://github.com/zaproxy/zaproxy/blob/master/src/xml/report.html.xsl - this is included in the XML directory.
Also happy to accept an issue raised for an html file option ;)
Btw - any reason why you're not using either zap.sh or zap.bat?
Cheers,
Simon