SelectFileWidget: Improve type checking
SelectFileWidget: Improve type checking

Use data transfer items in Chrome as these actually have mime type
information available. Change mime checker function to take strings
as we don't always pass Files.

Change-Id: I2c7fe8891912a44c6b681a3e27c83ffcb6015a48

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui