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().

deltachat-ffi/src/lib.rs

@@ -3362,6 +3362,34 @@ pub unsafe extern "C" fn dc_msg_get_file(msg: *mut dc_msg_t) -> *mut libc::c_cha
         .unwrap_or_else(|| "".strdup())
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_msg_save_file(
+    msg: *mut dc_msg_t,
+    path: *const libc::c_char,
+) -> libc::c_int {
+    if msg.is_null() || path.is_null() {
+        eprintln!("ignoring careless call to dc_msg_save_file()");
+        return 0;
+    }
+    let ffi_msg = &*msg;
+    let ctx = &*ffi_msg.context;
+    let path = to_string_lossy(path);
+    let r = block_on(
+        ffi_msg
+            .message
+            .save_file(ctx, &std::path::PathBuf::from(path)),
+    );
+    match r {
+        Ok(()) => 1,
+        Err(_) => {
+            r.context("Failed to save file from message")
+                .log_err(ctx)
+                .unwrap_or_default();
+            0
+        }
+    }
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_msg_get_filename(msg: *mut dc_msg_t) -> *mut libc::c_char {
     if msg.is_null() {