BitBucket server: configuration of syntax highlighting for file extensions not working

Added to Bitbucket\shared\bitbucket.properties:
"syntax.highlighter.text/javascript.extensions=abc"
and restarted BitBucket service.

Expected: my file test.abc in my BitBucket repo with js code is syntax highlighted.
Actual: no syntax highlighting

The bitbucket.properties is working for the database configuration, but syntax highlighting seems to be ignored. What can I do?
I am using BitBucket server 5.2.2.

Best regards,
Peter Bruch