fix(mail): Fix invalid content when forwarding a text/plain mail. Fix…

…es #5983.

SoObjects/Mailer/SOGoMailObject+Draft.m

@@ -134,9 +134,13 @@ - (NSString *) _contentForEditingFromKeys: (NSArray *) keys
           // Ticket https://bugs.sogo.nu/view.php?id=5983
           // In this case, the first HTML content is used, but it can be the previous forwarded mail
           // We check if there is a text/plain before text/html in the types array
-          // and if the key is not `body[1` (second part of the mail)
           // is this case, the text/plain is used prior to text/html.
-          if (index != NSNotFound && indexTextPlain != NSNotFound && indexTextPlain < index && [[[keys objectAtIndex: index] objectForKey:@"key"] rangeOfString:@"body[1"].location == NSNotFound)
+          if (index != NSNotFound // There is a text/html part
+              && indexTextPlain != NSNotFound  // There is a text/plain part
+              && indexTextPlain < index  // text/plain is before text/html
+              && (indexTextPlain + 1) < [keys count] // text/plain is not the last part of the mail
+              && [[[keys objectAtIndex: (indexTextPlain + 1)] objectForKey:@"mimeType"] rangeOfString:@"text/plain"].location != NSNotFound //  The text/plain is followed up by another text/plain
+              )
             index = indexTextPlain;
           else if (index == NSNotFound)
             index = [types indexOfObject: @"text/plain"];