As a security measure, most browsers including Firefox, IE, and Safari all do not allow a server-side value to be defaulted in an <input type=file> tag. It is therefore not possible to set a default value using the s:file tag as it is eventually rendered as an <input type=file> tag.
Think about it. If this were allowed, it would be possible for a malicious site to default the value of the file to the location of your Outlook file containing all your addresses and confidential information and upload it using JavaScript before you were even aware of what was happening.
This link Explains it.
[ October 21, 2008: Message edited by: Merrill Higginson ]