Yes, you can include any type of file, subject to a maximum total message size of 25MB. Messages that include attachments will be queued and the attachments processed through a series of virus scanning engines to help ensure that the attachments are safe for recipients.
Messages in Mandrill can't exceed 25MB in total size (including content and attachments). There is no specific limit for individual attachments, just the total size of the message. Since attachments are Base64-encoded, this generally means that they will be 1/3 larger when sending than they are on disk due to the encoding.