I've had a deeper look and it seems that the current version in github
is more recent than the gem version, and that in the github version
the default behaviour of the Attachment.url method has been changed to
append the date-time to the filename. However, the files aren't saved
with a datetime on the the end (by default) so the url that is
generated doesn't exist.
It seems like there are two solutions (long term):
* Change the default behaviour back to not including the datetime (I
think this one is most sensible as otherwise existing code will break
on upgrading to the latest version)
* Change the default interpolation for the file path to include the
datetime on the end of the filename (much cooler, but existing
projects will still be broken)