Function browser_mime_type 

pub fn browser_mime_type(name: &str) -> &'static str

The type a browser gives a file named name: by the extension after its last ., from the browser’s own table; "" for a name with no extension, or with one the table does not name.

The ONE mapping every tier that synthesizes a file from its name reads — a drop driven by a name alone, a File record that declares no type — so file's type on the tree-walker and the lowered VM is what a visitor’s browser reports for that name.