You need to add the -t FILESYSTEMTYPE argument to the command, replacing FILESYSTEMTYPE with your filesystem type. This specifies the filesystem type of the filesystem to be mounted. In your case, this would be /dev/sdb2. Some common, valid filesystem types are:
auto - this is a special one. It will try to guess the fs type when you use this.
ext4 - this is probably the most common Linux fs type of the last few years
ext3 - this is the most common Linux fs type from a couple years back
ntfs - this is the most common Windows fs type or larger external hard drives
vfat - this is the most common fs type used for smaller external hard drives