TF-2602 Set charset for attachment with mimeType=text/plain when sent email on mobile

Signed-off-by: dab246 <tdvu@linagora.com>

model/lib/extensions/attachment_extension.dart

@@ -11,7 +11,7 @@ extension AttachmentExtension on Attachment {
     name: name,
     type: type,
     cid: cid,
-    charset: charset,
+    charset: charset ?? this.charset,
     disposition: disposition?.name ?? ContentDisposition.attachment.name);
 
   Attachment toAttachmentWithDisposition({
@@ -25,7 +25,8 @@ extension AttachmentExtension on Attachment {
         name: name,
         type: type,
         cid: cid ?? this.cid,
-        disposition: disposition ?? this.disposition
+        disposition: disposition ?? this.disposition,
+        charset: charset
     );
   }
 

model/lib/extensions/email_body_part_extension.dart

@@ -9,5 +9,6 @@ extension EmailBodyPartExtension on EmailBodyPart {
     name: name,
     type: type,
     cid: cid,
+    charset: charset,
     disposition: disposition.toContentDisposition());
 }