fix: mkv videos not playing on browser

handlers.go

@@ -119,10 +119,18 @@ func (app *Application) indexHandler(w http.ResponseWriter, r *http.Request) {
 			".rst": true,
 		}
 
+		videoExtensions := map[string]bool{
+			".mkv": true,
+		}
+
 		if textExtensions[ext] {
 			w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 		}
 
+		if videoExtensions[ext] {
+			w.Header().Set("Content-Type", "video/mp4")
+		}
+
 		http.ServeFile(w, r, path)
 		return
 	}