set file extension for known file types using fourcc

src/main.rs

@@ -341,7 +341,13 @@ async fn download_list(cookie:String,asset_id_file_map:AssetIDFileMap)->AResult<
 	.buffer_unordered(CONCURRENT_REQUESTS)
 	.for_each(|b:AResult<_>|async{
 			match b{
-				Ok((dest,data))=>{
+				Ok((mut dest,data))=>{
+					//known file types
+					match &data[0..4]{
+						b"<rob"=>dest.set_extension("rbxm"),
+						b"\x89PNG"=>dest.set_extension("png"),
+						_=>false,
+					};
 					match tokio::fs::write(dest,data).await{
 						Err(e)=>eprintln!("fs error: {}",e),
 						_=>(),