feat: Don't put text into post-message (#7714)

Before this PR, when a user with current main sends a large message to a
user with an old Delta Chat (before #7431), the text will be duplicated:
One message will arrive with only the text, and one message with
attachment+text.

This PR changes this - there will be one message with only the text, and
one message with only the attachment.

If we want to guard against lost pre-messages, then we can revert this
PR in a few months, though I'm not sure that's necessary - it's unlikely
that the small pre-message gets lost but the big post-message gets
through.

src/mimefactory.rs

@@ -1872,19 +1872,23 @@ impl MimeFactory {
 
         let footer = if is_reaction { "" } else { &self.selfstatus };
 
-        let message_text = format!(
-            "{}{}{}{}{}{}",
-            fwdhint.unwrap_or_default(),
-            quoted_text.unwrap_or_default(),
-            escape_message_footer_marks(final_text),
-            if !final_text.is_empty() && !footer.is_empty() {
-                "\r\n\r\n"
-            } else {
-                ""
-            },
-            if !footer.is_empty() { "-- \r\n" } else { "" },
-            footer
-        );
+        let message_text = if self.pre_message_mode == PreMessageMode::Post {
+            "".to_string()
+        } else {
+            format!(
+                "{}{}{}{}{}{}",
+                fwdhint.unwrap_or_default(),
+                quoted_text.unwrap_or_default(),
+                escape_message_footer_marks(final_text),
+                if !final_text.is_empty() && !footer.is_empty() {
+                    "\r\n\r\n"
+                } else {
+                    ""
+                },
+                if !footer.is_empty() { "-- \r\n" } else { "" },
+                footer
+            )
+        };
 
         let mut main_part = MimePart::new("text/plain", message_text);
         if is_reaction {