workflows: don't return attachment contents for remote opaque files (#53842)

wcs/workflows.py

@@ -279,8 +279,10 @@ class AttachmentEvolutionPart:
             'content_type': self.content_type,
             'filename': self.base_filename,
         }
-        with open(self.filename, 'rb') as fd:
+        fd = self.get_file_pointer()
+        if fd:
             d['content'] = base64.encodebytes(fd.read())
+            fd.close()
         return d
 
     @classmethod