prevent mac from renaming downloads' extensions Sometimes I download something, and OS X (or Chrome, not sure) decides that the file extension must be changed. 

Is it possible to completely disable this feature? How?

I guess this is a Google Chrome issue, but triggered in OS X because it hides file extensions by default? I'm not sure how it works and what triggers this, but I'd just like to have the downloaded file's name not to be messed with.

You're offered the option of which extension to use, but sometimes, even if you choose the right extension, it still screws up the filename.
Go to the Chrome menu and select preferences. Scroll to the bottom of the preference window and click on Advanced. This lets you scroll down to a 2nd page off options that you can use or turn off. Two of the first options refer to using a Prediction service for loading web pages. You can try turning one or both of these off to get rid of your problem.