Purpose of the --forward flag and difference between it and --to for telegram-upload?

[sondhg]

I have a private channel of my own. Let's say its ID is -123.

telegram-upload a.jpg uploads the file to my Saved messages.
telegram-upload a.jpg --to '-123' a.jpg uploads to that channel I mentioned.

What if I want to upload the file to both my Saved messages and that channel? Do I have to upload to Saved messages, then manually select and forward the message to the channel? I don't know if it's possible to upload to both places at once using telegram-upload or not.

I thought the-f or --forward flag was for this use case. Documentation says:

-f, --forward <forward>
Forward the file to a chat (alias or id) or user (username, mobile or id). This option can be used multiple times.

so I tried telegram-upload --forward '-123' a.jpg. But an error occurred:

ValueError: Cannot find any entity corresponding to "-123"

I'm confused about the difference between --forward and --to too.
Am I understanding something wrong? What should I do? Thank you.