Don't send "Content-Type: none"

mimetypes.guess_type will return (None, None) when it can't detect the
filetype.

mercurial/hgweb.py

@@ -419,7 +419,8 b' class hgweb(object):'
         mt = mimetypes.guess_type(f)[0]
         rawtext = text
         if util.binary(text):
-            text = "(binary:%s)" % mt
+            text = "(binary:%s)" % (mt or 'data')
+        mt = mt or 'text/plain'
 
         def lines():
             for l, t in enumerate(text.splitlines(1)):