Improve comment on RFC 1867 filename handling

Improve comment on RFC 1867 filename handling

It was pointed out on the internals ML[1] that the basename comment
around RFC 1867 file handling was confusing. The comment seemed to
say that the basename handling was only necessary because of some
esoteric bug in Windows IE. In fact that comment applied to earlier
versions of the code and was, therefore, misleading. The comment
needs to clear that basename is required per RFC 7578[2].

[1]:https://externals.io/message/108624
[2]:https://tools.ietf.org/html/rfc7578