Look at the MacBinary file that is created, and you'll notice that byte $4A is non-zero, which is incorrect, according to the MacBinary Standard. Also, byte $49, which should contain some of the Finder flags, usually seems to contain incorrect information. Setting byte $4A to zero seems to fix the problem--the file will then successfully download. Note that when creating a MacBinary from an AppleSingle file, these two bytes seem to be okay and the file downloads okay.