I have Mint 16 MATE. The file browser is Caja. If you have a different flavor of Mint or file browser this may not help.
In the file browser, click Edit in the menu bar and then click Preferences.
In File Management Preferences, click the Preview tab at the top.
I have under Other Previewable Files, Show Thumbnails.
The options are Always, Never, and Local Files Only.
Choose either Always or Local Files Only.