I cannot see an upload button in chat.
Why is this? Maybe you require a certain amount of reputation to get this feature. How much do you need?
The fixed font button is missing, too.
Perhaps it should be listed as a privilege in this page.
You need 100 reputation to upload images directly. You're right, this doesn't seem to be documented anywhere, we should fix this.
As for the "fixed font" button, that only appears when you have multiple lines of content in the chat box. It's for e.g. pasting a log file, some source code (as in your example screen shot), or similar, which you usually want to display in a monospace font.