if dest_path is not set, the file will be downloaded in cache. A new name will be
generated using a hash of the uri. If the file is already downloded in cache, it won't be
downloaded again (in this case returned progress_id is an empty string).
- first code in backend to use async/await Python syntax \o/
- plugin with file upload
- URL schemes can be registered
- `http` and `https` schemes are handled by default
