Attachment_fu not setting s3_access on thumbnails?

I’m checking out attachment_fu and S3, and I’m able to display
uploaded images nicely. However, requests for generated thumbnails are
getting “access denied” from Amazon. My guess is that the thumbnails’
s3_access attribute is not being set to public_read, as is the default
on the original file.

What would be the simplest approach to make this the default setting
for attachment_fu thumbnails as well?