guess MIME for JavaScript files so that `ptth_file_server` can serve Jet Racing 4 locally.

Firfox has a bunch of odd security features that I only sometimes understand.
Among them is that it won't load JS files without the `content-type` header,
and it also disables a lot of features for the `file://` scheme. Which I think
is a shame. There's probably a good reason they do this, but I'm not aware of one.

So now you can use PTTH's file server to host Jet Racing 4.

crates/ptth_server/src/file_server.rs

@@ -141,6 +141,7 @@ async fn serve_dir_json (
 
 #[instrument (level = "debug", skip (f))]
 async fn serve_file (
+	uri: &str,
 	mut f: File,
 	client_wants_body: bool,
 	range: range::ValidParsed,
@@ -211,6 +212,12 @@ async fn serve_file (
 		response.header (String::from ("content-range"), format! ("bytes {}-{}/{}", range.start, range.end - 1, range.end).into_bytes ());
 	}
 	
+	// Guess MIME type based on the URI so that we can serve web games
+	
+	if uri.ends_with (".js") {
+		response.header ("content-type".into (), b"application/javascript".to_vec ());
+	}
+	
 	response.content_length = Some (content_length);
 	
 	if let Some (body) = body {
@@ -402,7 +409,7 @@ impl FileServer {
 				file,
 				send_body, 
 				range, 
-			}) => serve_file (file.into_inner (), send_body, range, headers.get ("if-none-match").map (|v| &v[..])).await?,
+			}) => serve_file (uri, file.into_inner (), send_body, range, headers.get ("if-none-match").map (|v| &v[..])).await?,
 			MarkdownErr (e) => {
 				#[cfg (feature = "markdown")]
 				{