Clang-format on save does not work for headers

I have set up beautifier to to autoformat files on save. This only works for source files, not headers. Further details:

• Using the Meson project plugin
• Beautifier set to clang-format, style "file", format on save
• Running Tools -> beautifier -> clang-format -> format current file works
• The mime type list does contain text/x-c++hdr