command line - Difficulty trying to mount an ISO file "mount: you must specify the filesystem type"

I have an .iso file which I would like to mount. I have made a folder in /mnt/iso as a mount point. Now I try:

sudo mount -o loop filename.iso /mnt/iso

and the reply is:

mount: you must specify the filesystem type

and the folder of /mnt/iso is empty. I have tried using

sudo mount -o loop -t ext4 filename.iso /mnt/iso

it says that mount: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error (and more text)...

I found the filesystem type from using df -T and mount at the command line.

What could be missing or wrong?

The filetype you need for mounting a .iso formatted file is iso9660. Here's an example:

A good .iso:

walt@wombat:~(0)$ file  /usr/lib/memtest86+/memtest86+.iso
/usr/lib/memtest86+/memtest86+.iso: # ISO 9660 CD-ROM filesystem data  MT410 ' (bootable)  `

A directory to mount it on:`

walt@wombat:~(0)$ ls -ld ~/tmp/mp
drwxrwxr-x 2 walt walt 4096 2012-01-01 13:55 /home/walt/tmp/mp`

The actual mount:

walt@wombat:~(1)$ sudo mount -t iso9660 /usr/lib/memtest86+/memtest86+.iso /home/walt/tmp/mp
mount: warning: /home/walt/tmp/mp seems to be mounted read-only.`

And here it is!

walt@wombat:~(0)$ ls !$
ls /home/walt/tmp/mp
boot  readme.txt
walt@wombat:~(0)$  df -H /home/walt/tmp/mp
Filesystem             Size   Used  Avail Use% Mounted on
/dev/loop0             1.9M   1.9M      0 100% /home/walt/tmp/mp
walt@wombat:~(0)$