[Question] Telegram channel file icons?

[Sakujes]

The official @forkgram telegram channel is the only one I have ever seen to have files with icons on Telegram, and since it has no discussion group or other place to talk, I need to ask here:
How are the file icons added before they are uploaded to the Telegram channel? Is there any 3rd party tool able to do this to any file? I tried downloading the latest https://github.com/Forkgram/TelegramAndroid/releases/download/8.0.0.0/ForkClient_8.0.0.0.apk and uploading that straight to Telegram, but it did not have the custom icon below:

[Sakujes]

For anyone else looking, it is answered here: forkgram/tdesktop#54 (comment) (thumb Parameter)