Mattermost 3.1 (releasing June 16) offers keyboard shortcuts to help you navigate through channels and make a more efficient use of your keyboard, including
CTRL/CMD + K: Open channel switcher
Alt + Shift + Up/Down: Previous/Next channel or direct message with unread messages.
CTRL/CMD + U: Upload a file
To see all available keyboard shortcuts, check out our docs
Big thank you to everyone who shared feedback, forum requests, uservoice voting and contributions toward this feature!
If you would like to request additional shortcuts, you can do so at our Feature Idea forum.