Value of Content-Disposition header:
Content-Disposition: attachment; filename="foo-%c3%a4-%e2%82%ac.html"
'attachment', specifying a filename of
foo-%c3%a4-%e2%82%ac.html, using raw percent encoded UTF-8
UA should offer to download the resource as "foo-%c3%a4-%e2%82%ac.html". Displaying "foo-ä-€.html" instead would indicate that the UA has attempted to percent-decode the parameter (using UTF-8). Displaying something else would indicate that the UA tried to percent-decode, but used a different encoding.