For Buffers, set the content type to application/octet-stream by default.

When buffers are attached to a form they were added with no content type; therefore, the content type would default to 'text/plan'. This modification changes this behaviour to default to 'application/octet-stream' for buffers.
The user is also able to add a 'filename' optional field to the header in case a file is being sent in binary form.
Do we really need to have this header?