fix: get_filename() is now guaranteed to return a valid filename (#6537)

With iOS and Desktop copying the file to a to a temp file with the name
of `get_filename()`, it should be sanitized first.

The PR can be reviewed commit-by-commit or all at once.

src/message/message_tests.rs

@@ -755,3 +755,31 @@ async fn test_delete_msgs_offline() -> Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_sanitize_filename_message() -> Result<()> {
+    let t = &TestContext::new().await;
+    let mut msg = Message::new(Viewtype::File);
+
+    // Even if some of these characters may be valid on one platform,
+    // they need to be removed in case a backup is transferred to another platform
+    // and the UI there tries to copy the blob to a file with the original name
+    // before passing it to an external program.
+    msg.set_file_from_bytes(t, "/\\:ee.tx*T ", b"hallo", None)?;
+    assert_eq!(msg.get_filename().unwrap(), "ee.txT");
+
+    let blob = msg.param.get_blob(Param::File, t).await?.unwrap();
+    assert_eq!(blob.suffix().unwrap(), "txt");
+
+    // The filename shouldn't be empty if there were only illegal characters:
+    msg.set_file_from_bytes(t, "/\\:.txt", b"hallo", None)?;
+    assert_eq!(msg.get_filename().unwrap(), "file.txt");
+
+    msg.set_file_from_bytes(t, "/\\:", b"hallo", None)?;
+    assert_eq!(msg.get_filename().unwrap(), "file");
+
+    msg.set_file_from_bytes(t, ".txt", b"hallo", None)?;
+    assert_eq!(msg.get_filename().unwrap(), "file.txt");
+
+    Ok(())
+}