Attachment names are made up of the following strings, taken directly from the email attachment item:
SentBy_DateSent_AttachmentName.AttachmentExtension
SentBy = The person who sent the original email item (with attachment).
DateSent = The date and time the original email item (with attachment) was sent to you. The format for this field is MM-DD-YYYY_hhmmss.
AttachmentName = The Attachment's file name as it is stored in your email.
AttachmentExtension = The Attachment's file extension as it is stored in your email.