BW::HTTP file upload -- fixed content-disposition

This took me several hours to figure out tonight. When I finally created my own implementation, I realized that the content-disposition was set incorrectly.

File attachments should be set to a Content-Disposition of "attachment", not "form-data". Reference: http://tools.ietf.org/html/rfc2183

I also added a much-needed file upload example to the README.