False. If you want to tell how things works, get your facts right!
All data sent to Telegram's servers will be encrypted once they reach the servers. With other words, the messages and media and other files, will be sent in "plain text" over HTTPS only when using Cloud Chat. In Secret Chat, MTProto is (based on how E2EE works) as safe as what Signal Protocol is.
But nothing will be stored in plain text, no matter what you use (Cloud Chat or Secret Chat).
But(!) since the source code for MTProto is closed, we don't know how it really works, and if we can trust their FAQ or not.
I trusted Telegram at first, but I don't trust it 100% anymore (still better than SMS). Am using my own Snikket server these days. Much safer with a lot of 😌😊😍😃 moments, even today, maybe a year later. Especially with OMEMO (Signal Protocol).