[UPDATE: there is no update. This is still an issue in Yosemite]
I've had an ongoing hate affair with OSX's Finder app, which is so inferior to Windows Explorer as to make me want to switch back on a daily basis. It's terrible, and one of the bugs that has been annoying me most recently is the Case of the Missing Screenshot.
I take screenshots all the time - our Trello cards are littered with them, and cmd+shift+4 is about the only three-finger shortcut on a Mac I can remember. (On an unrelated note I recently discovered four-finger shortcuts - WTF - that isn't a shortcut, it's a form of Yoga.)
The problem has come when I take a screenshot, click on the add attachment button in Trello (that's where I see the problem most often, albeit not the only place) only to find that the screenshot isn't showing up in the Finder Open File dialog as expected. If I use Spotlight I can find it, if I open Finder and browse to the folder I can find it, but when opening it from within an app I can't. What gives?
There are lots of threads on lots of forums (fora?) about this, most of them recommend either killing
com.apple.Finder.plist, or downloading RefreshFindr. Neither of these help.
The first solution I found is the nuclear option, with a suitably militarty sounding explanation (Resetting the NVRAM). - and involves one of those four-finger shortcuts - cmd+alt+p+r. To quote from the answer:
I got the problem temporarily fixed by resetting NVRAM (PRAM):
- restart your Mac
- during startup press alt-cmd-p-r until you hear a second chime
The alt-cmd-p-r key combination is know as "zapping the NVRAM / PRAM". The PRAM is a programmable chip that holds various bits of information about your computer's hardware along with other items. It's role is similar to a BIOS chip in a typical PC.
Honestly, that isn't really a solution - and you'd have to be really desperate (I was) to do it.
So it looks like the problem is related to the way in which applications get / hold the creation dates of files. Hard to believe, but apparently if you have an app open, it won't recognise files created in the future - so if you open Chrome and leave it open overnight, anything created on / after the second day won't appear in the Open File dialog. Solution is to restart Chrome. Every day.