The original files and resulting fax images are kept for 3 days. This was originally done to allows us to investigate reports of quality issues upon request by customers but is probably is no longer necessary.
A low level shredding operation would make me more comfortable, especially since the user has no way to access the attachment nor to even get a scaled image of page 1 like hardware faxes.
While I appreciate that sentiment I would suggest that if you feel that you need this level of security from your faxes that you should reconsider using ours (or anyone's) e-faxing services. We'd certainly never shred queue files - a simple unlink() is sufficient for our needs. I'll disable the preservation of the faxes the next time I make a release.