Try the usual things first. Are you doing this on a regular computer/laptop, or from your phone? Sometimes certain actions can get a little squirrely from our phones. Also, check your file size to make sure it's not way out of bounds. If it's within bounds, then clear your cache, then try again to attach it. If that doesn't help, then switch browsers.
It's likely one of these things, or a combination, that's causing the hiccup. Nothing widespread is being reported, so more than likely it's something like the above on your end that's the culprit.