A .tmp file is usually a temporary file (in Windows or Mac). It may be something that a program or process creates "in progress," but it is not the final product. So, it may be related to creating or processing a video, but it is not the final video file.
That is why Mac OS X is asking what you want to do with this file? There is no application designated to open a .tmp file.
If you think it's a video file that has the wrong extension, you can right-click it (or click it while holding down Control key) and select
Open With -> QuickTime Player
If it's a video file, QuickTime Player probably has the best shot of opening it, out of applications that come pre-installed.
And if you're new to the Mac world, this resource from Apple may be helpful
http://www.apple.com/support/macbasics/