Additional field to the File structure (#1370)

Slack has a feature called 'Send emails to Slack' -
https://slack.com/intl/en-gb/help/articles/206819278-Send-emails-to-Slack

When emails are sent to this channel, they become Slack files with
additional fields containing sender and receiver information.

I added two new fields to properly support the unmarshaling of file
metadata.

files.go

@@ -89,6 +89,17 @@ 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"`
+	Cc      []EmailFileUserInfo `json:"cc"`
+}
+
+type EmailFileUserInfo struct {
+	Address  string `json:"address"`
+	Name     string `json:"name"`
+	Original string `json:"original"`
 }
 
 type Share struct {