A piece of code I wrote for myself to transfer all saved messages, files and photos from "Saved messages" chat on my old account to "Saved messages" chat on my new account.

Course of action:

• Install TDLib (the main Telegram API for building clients). There are binaries floating around if you don't want to build everything from scratch.
• Install python-telegram (python wrappers).
• Do JSON export of all your personal chats with photos and files if needed (settings -> advanced -> export Telegram data on Telegram 1.9.21). You get result.json and chats, put them in the script directory and run json_to_messages.py to single "saved messages" out and to convert it to a format ready to be sent. Unfortunately, text formatting gets cut, because I just wanted to retrieve plaintext, photos and files.
• Go to Telegram developer portal and register a new app to get api_id and api_hash (very quick). Enter api_id, api_hash and your phone number in send.py.
• If you use Linux and have TDLib in PATH, remove library_path parameter in send.py. If you use Windows, dump TDLib dll binaries in script directory and provide a path to tdjson.dll.
• Turn VPN on if Telegram is blocked in your country.
• Run send.py, log in with a code and check that the messages are flowing. It may take time for all of them to go because of intentional delay between messages (to protect the flow against API anti-spam delays). I've noticed that TDLib error handlers do not work as I expected them to work, so makes sense to observe terminal error messages too.