I have this issue (or a very similar issue) on Karmic as well:
$ file --brief --mime ubuntu-9-10-rc-desktop-amd64.iso application/octet-stream; charset=binary
$ file ubuntu-9-10-rc-desktop-amd64.iso ubuntu-9-10-rc-desktop-amd64.iso: ISO 9660 CD-ROM filesystem data 'Ubuntu 9.10 amd64 ' (bootable)
I have this issue (or a very similar issue) on Karmic as well:
$ file --brief --mime ubuntu- 9-10-rc- desktop- amd64.iso octet-stream; charset=binary
application/
$ file ubuntu- 9-10-rc- desktop- amd64.iso 9-10-rc- desktop- amd64.iso: ISO 9660 CD-ROM filesystem data 'Ubuntu 9.10 amd64 ' (bootable)
ubuntu-