Inline attachments

[jlazic]

This commit adds new argument that enables inline attachment in Content-Disposition. This enables user to have file displayed inline and force filename via attachment_filename.

[vstoykov]

I'm wondering if it will be better to use:

return sendfile(request, '/home/john/myfile.pdf', inline=True, attachment_filename='myfile.pdf')

Instead of passing both attachment and inline.