feat: add default outputs to meta plugins

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/meta_plugin/magic.rs

@@ -145,5 +145,9 @@ impl MetaPlugin for MagicFileMetaPlugin {
     fn outputs_mut(&mut self) -> &mut std::collections::HashMap<String, serde_yaml::Value> {
         &mut self.outputs
     }
+    
+    fn default_outputs(&self) -> Vec<String> {
+        vec!["mime_type".to_string(), "mime_encoding".to_string(), "file_type".to_string()]
+    }
 }