Fix so multipart buffer string is explicitly created as BINARY/ASCII-8BIT

Error during file upload was:
    ArgumentError: invalid byte sequence in UTF-8
kernel/common/string.rb:103:in `=~'

/app/vendor/bundle/rbx/2.1/gems/rack-1.4.5/lib/rack/multipart/parser.rb:
92:in `get_current_head_and_filename_and_content_type_and_name_and_body'

/app/vendor/bundle/rbx/2.1/gems/rack-1.4.5/lib/rack/multipart/parser.rb:
19:in `parse'