Okay, Say you have a Wubi‘s root.disk image that you want to mount it on a Linux system in order to access its files and contents. Well, here is what you have to do:

First, become root:

$ su

Or, you can use sudo as a prefix to the following commands.

Second, prepare the mount-point folder:

A mount point is a directory (typically an empty one) in the currently accessible filesystem on which an additional filesystem is mounted (i.e., logically attached). A filesystem is a hierarchy of directories (also referred to as a directory tree) that is used to organize files on a computer system. Source

$ mkdir -p /mnt/wubi

Keep in mind that the /mnt/ path can be anywhere else as you like. Same thing would apply to the name of mount-point, which in our case is wubi.

Then, mount the the root.disk:

$ mount -o loop /path/to/the/root.disk /mnt/wubi

Finally, if you want to un-mount it, simply do:

$ umount /mnt/wubi