files: fix raw download of repo files with names with unicode points above 256 in name

Raw download had apparently only been tested with non-ascii characters that
were latin1. That was apparently a (too) simple case that worked without
crashing.

Files with unicode code points above 256 in their name would fail to download,
when Waitress failed like this, trying to get a real byte string by encoding
WSGI headers to latin1:
UnicodeEncodeError: 'latin-1' codec can't encode characters in position 84-85: ordinal not in range(256)

HTTP headers are of course byte strings on the network, but Python3 WSGI does
unfortunately neither expose it as bytes nor as unicode strings to be encoded
as utf-8. Instead, it uses unicode strings with byte values encoded as code
points 0-255. That is achieved by decoding the utf-8 encoded bytes as latin1.

For raw downloads, the recommended download filename is provided in the
Content-Disposition header. The problem is that it was provided as a real
unicode string.

Fixed by applying the "proper" latin1-decoding of a utf8-encoding.