api: Add dc_msg_save_file() which saves file copy at the provided path (#4309)

... and fails if file already exists. The UI should open the file saving dialog, defaulting to
Downloads and original filename, when asked to save the file. After confirmation it should call
dc_msg_save_file().

src/message.rs

@@ -6,6 +6,7 @@ use std::path::{Path, PathBuf};
 use anyhow::{ensure, format_err, Context as _, Result};
 use deltachat_derive::{FromSql, ToSql};
 use serde::{Deserialize, Serialize};
+use tokio::{fs, io};
 
 use crate::blob::BlobObject;
 use crate::chat::{Chat, ChatId};
@@ -605,6 +606,19 @@ impl Message {
         self.param.get_path(Param::File, context).unwrap_or(None)
     }
 
+    /// Save file copy at the user-provided path.
+    pub async fn save_file(&self, context: &Context, path: &Path) -> Result<()> {
+        let path_src = self.get_file(context).context("No file")?;
+        let mut src = fs::OpenOptions::new().read(true).open(path_src).await?;
+        let mut dst = fs::OpenOptions::new()
+            .write(true)
+            .create_new(true)
+            .open(path)
+            .await?;
+        io::copy(&mut src, &mut dst).await?;
+        Ok(())
+    }
+
     /// If message is an image or gif, set Param::Width and Param::Height
     pub(crate) async fn try_calc_and_set_dimensions(&mut self, context: &Context) -> Result<()> {
         if self.viewtype.has_file() {