Summary
When you copy text for example from a website and try to paste it into your mattermost chat line, it tries to format it. Its not only utterly useless, goes against the default behavior of 99% of applications in the world, but also broken most of the time.
Can it be disabled?
Steps to reproduce
Copy and paste from a website into mattermost chat.
it cannot be disabled, but you can use the unformatted paste keyboard shortcut which is standard nowadays in most of the applications, that is CTRL/Cmd-SHIFT-v instead of just CTRL/Cmd-v.