Feature (canvas): Add Api for download "message" component output's file (#11772)

### What problem does this PR solve?

-Add Api for download "message" component output's file 
-Change the attachment output type check from tuple to
dictionary,because 'attachement' is not instance of tuple
-Update the message type to message_end to avoid the problem that
content does not send an error message when the message type is ans
["data"] ["content"]

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

agent/canvas.py

@@ -534,10 +534,12 @@ class Canvas(Graph):
                         yield decorate("message", {"content": cpn_obj.output("content")})
                         cite = re.search(r"\[ID:[ 0-9]+\]",  cpn_obj.output("content"))
 
-                    if isinstance(cpn_obj.output("attachment"), tuple):
-                        yield decorate("message", {"attachment": cpn_obj.output("attachment")})
-
-                    yield decorate("message_end", {"reference": self.get_reference() if cite else None})
+                    message_end = {}
+                    if isinstance(cpn_obj.output("attachment"), dict):
+                        message_end["attachment"] = cpn_obj.output("attachment")
+                    if cite:
+                        message_end["reference"] = self.get_reference()
+                    yield decorate("message_end", message_end)
 
                     while partials:
                         _cpn_obj = self.get_component_obj(partials[0])