Version: 52.0.2730.0 (Official Build) canary (64-bit)
OS: Mac OS X

What steps will reproduce the problem?
(1) Edit a message in Gmail.
(2) Click "Attach files".

What is the expected output?

Expect the file chooser drop-down to appear, to select an attachment.

What do you see instead?

Nothing happens, and no errors are logged to the JavaScript console.

I haven't bisected this yet, but the behavior persists across browser restarts.