Workflow email provides mapping of fields where fileattach , filename are need to map. Fileattach has to provide the full path with file location and Filename has to map with the display name in email for attachment.
If you have to pick file from File server then simply you can give the file path from the fileserver.
If you have file in database record as attachment, then get that attachment using getattachment) function and store the file in some fileserver and then provide the exact file path in Workflow email.

Hope it will clear your doubts.