Fix insecure extension check in read_sheet_file

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

src-tauri/src/main.rs

@@ -938,8 +938,12 @@ fn load_extensions(app: AppHandle) -> Vec<ExtensionScript> {
 }
 
 fn is_sheet_path(path: &str) -> bool {
-    let lower = path.to_lowercase();
-    lower.ends_with(".numi") || lower.ends_with(".sum")
+    std::path::Path::new(path)
+        .extension()
+        .map_or(false, |ext| {
+            let ext = ext.to_string_lossy().to_lowercase();
+            ext == "numi" || ext == "sum"
+        })
 }
 
 /// A sheet handed to us by the OS (file association, second launch): same