Added AttachmentType to Attachment.cs to differentiate between Attachments and Inline-Attachments

I wanted to filter out inline attachments in my application. There was no proper way of doing this yet and I have built it in. Could not find any more content-disposition types besides 'attachment' and 'inline'. If it is not find, it default to 'Unknown'.

Added AttachmentType to Attachment.cs to differentiate between Attachments and Inline-Attachments
Added UnitTests