This morning I had the same problem and discovered how to get my GS6 to let me move files between it and my PC. I think there must've been a firmware update to change the usual behavior. After you plug in the USB cable, check for a notification on the phone that says "Connected for charging. Touch for other USB options." Tapping on it will give you a dialog box to enable moving files, images, etc. I guess it's for improved security.
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