#882 unobtrusive validation - support extension() and accept() methods of jquery validation

in version 1.10 of jquery validation, the old accept() method (for file extensions) was replaced by extensions() method,
while accept() is now validating the file's content type.

a backward compatiblity logic is presented to backport html attributes produced by the new code, to be compatible with a site that uses an older jquery.validator plugin
#1040 Empty SingleResult action should return 404