If you haven't used Google Drive (the web interface, not the software you install on your computer), and you're just about to (come on, everybody's doing it!), or you haven't discovered this setting yet, here is a quick way to avoid hassle down the road:
Step 1: Trust me on this one, ok?
Step 2: In Google Drive, click the little gear in the upper right, and then click on "Settings"
Step 3: Click the "Convert uploaded files to Google Docs editor format" checkbox, and then the blue "Done" button: