Long time users of WiFi File Explorer may remember back in the olden days where you could actually write to the SD card in your Android device - crazy stuff, huh? You'll also remember when Android 4.4 was released and ruined that blissful utopia, taking away SD card write access, meaning a lot of file operations in WiFi File Explorer (and other file-based apps) stopped working as they used to, like upload, delete, rename, etc.
Android 5.0 addresses this to a degree and once again allows SD card write access to third party apps, albeit in a rather convoluted way. The user must grant an app access to a particular area of storage using a folder chooser dialog which is presented via the Android OS. I'll take you through those steps in relation to WiFi File Explorer so that you can once again manage the SD card using the app. Remember though, this only applies to devices running Android 5.0+ and the latest version of WiFi File Explorer; it will not fix the issue on Android 4.4 devices.