Pulled from VS Code official repository which itself pulls from Seti UI repository. It's possible to manually edit the vs-seti-icon-theme.json file located in $VSCODE_PATH/resources/app/extensions/theme-seti/icons/vs-seti-icon-theme.json. WithVSCODE_PATH being the installation folder. In Linux defaults to /usr/share/code/
Anytime you wonder which and where were all the file.associations for customizing the file extension icons in VS Code? Well here they are.
Your settings.json would look the following example (first one being place holders and the second one being a working icon file association)
"files.associations": {
"REGEX_PATTERN": "FILE.ASSOCIATION",
"*.xcu": "xml"