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