How do I force OSX to always save file extension? When saving a file without an extension not even OS X is able to open that file anymore. Still it is very easy to accidentally save a file without an extension.
Example: When using Firefox and you save an image with the "Save image as..." context menu, the whole filename including the extension is marked for renaming and as soon as I entered a custom name the extension is gone and I have to reenter it.

How do I solve this problem?
You can hide common filename extensions, so in the save dialog no filename extension will be shown.
In a finder window, open preferences from Finder menu. Uncheck Show all filename extensions.