Configuring syntax highlighting for custom file extensions not working

Summary

Configuring syntax highlighting for custom file extensions does not take effect

Steps to Reproduce

1. In bitbucket.properties add an entry for extension e.g. for .csproj syntax.highlighter.application/xml.extensions=csproj
2. Restart Bitbucket server
3. Add a file with .csproj extension and some XML content
4. Browse the file

Expected Results

Content is highlighted as https://codemirror.net/mode/xml/index.html

Actual Results

Content is not highlighted

Note: This issue has also been seen to affect standard file extension detection, where the wrong syntax highlighting may be seen to be applied to standard file extensions (e.g. Objective-C may have Mathematica highlighting incorrectly applied)