Fix crash on start-up introduced by 766534fa0
Fix crash on start-up introduced by 766534fa0

Caused when mime-type look-up fails.

MobileChromeApps/cordova-plugin-chrome-apps-bootstrap,DanielJoyce/cordova-plugin-chrome-apps-bootstrap,DanielJoyce/cordova-plugin-chrome-apps-bootstrap,MobileChromeApps/cordova-plugin-chrome-apps-bootstrap,DanielJoyce/cordova-plugin-chrome-apps-bootstrap,MobileChromeApps/cordova-plugin-chrome-apps-bootstrap