For the current Server Mail event template, you can only attach documents that:
1. Are somewhere on the local file system of the K2 Server machine
2. Are kept as a binary file in a Datafield

To get the required document, you can add a SharePoint 2003 Document event before the mail event that sends the mail, select Get Latest Version as your action and select a K2.net datafield in the Destination Document Setup screen. Lastly you ll have to configure the Server Mail event to get the attachment from a K2.net Field, specify the field name, Temp Directory to be used for creating the physical doc to attach and a document name.

Couple of considerations:
1. When you create the binary datafield to keep the doc, check Hidden Field (since the file is kept as a binary user wont make any sense of it anyways) and uncheck Keep Audit Trail .
2. Attachments can be quite big, so I would suggest clearing the datafield used for keeping the binary field after sending the email. I m not sure if the temp document gets deleted after the email is send, you may want to do clean up afterwards if not done automatically.
3. The account used to run your K2 Server will need permissions on the temporary folder used to create doc to be attached, ensure that it is given.

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