Additional Headers field to the File structure (#1380)

add Headers field to support headers of an email file attachment.

Example: 
```
"headers": {
    "date": "Thu, 23 Jan 2025 18:09:25 +0100",
    "in_reply_to": null,
    "reply_to": null,
    "message_id": "<email_provider_message_id>"
},
```

related to #1370

Author: nlopes

files.go

@@ -94,6 +94,7 @@ type File struct {
 	To      []EmailFileUserInfo `json:"to"`
 	From    []EmailFileUserInfo `json:"from"`
 	Cc      []EmailFileUserInfo `json:"cc"`
+	Headers EmailHeaders        `json:"headers"`
 }
 
 type EmailFileUserInfo struct {
@@ -102,6 +103,13 @@ type EmailFileUserInfo struct {
 	Original string `json:"original"`
 }
 
+type EmailHeaders struct {
+	Date      string `json:"date"`
+	InReplyTo string `json:"in_reply_to"`
+	ReplyTo   string `json:"reply_to"`
+	MessageID string `json:"message_id"`
+}
+
 type Share struct {
 	Public  map[string][]ShareFileInfo `json:"public"`
 	Private map[string][]ShareFileInfo `json:"private"`