[BUGFIX] Send different download headers for IE browsers

This fixes problems with downloading passthrough files using
the UriWidget when browser is IE. By sending additional download
headers, some different from the norm, IE can also download files.

Change-Id: I13fd05783a784f9d7170a0745e88e4e96d17e6df
Merge pull request #3 from FluidTYPO3/ieheaders

[BUGFIX] Send different download headers for IE browsers