add subject File field for email file

files.go

@@ -89,6 +89,7 @@ type File struct {
 	NumStars        int                 `json:"num_stars"`
 	IsStarred       bool                `json:"is_starred"`
 	Shares          Share               `json:"shares"`
+	Subject         string              `json:"subject"`
 	To              []EmailFileUserInfo `json:"to"`
 	From            []EmailFileUserInfo `json:"from"`
 }