Binaries servlet generates invalid encoded filename for content disposition header

A PDF binary served by the BinaryServlet fails to open with IE6. For exapmle, we get the filename:

=UTF-8_B_Zm9ybXVsaWVyLWFhbnZyYWFnLXRvdC1zdWJzaWRpZXY==_=UTF-8_B_YXN0c3RlbGxpbmctdjA5LXRjbTE5LTE2MDY2MC5wZGY==

This somehow makes it impossible to download or open the binary.