When a file has the i attribute set, the only thing you can do with it is view its contents. You can't change it, move it, delete it, rename it, or create hard links to it. Let's test this with the perm_demo.txt file:

[donnie@localhost ~]$ sudo chattr +i perm_demo.txt
[donnie@localhost ~]$ lsattr perm_demo.txt
----i----------- perm_demo.txt
[donnie@localhost ~]$

Now for the fun part:

[donnie@localhost ~]$ sudo echo "I want to overwrite this file." > perm_demo.txt
-bash: perm_demo.txt: Permission denied
[donnie@localhost ~]$ echo "I want to append this to the end of the file." >> perm_demo.txt
-bash: perm_demo.txt: Permission denied
[donnie@localhost ~]$ sudo echo "I want to append this to the end of the file." >> perm_demo.txt
-bash: perm_demo.txt: Permission denied
[donnie@localhost ~]$ rm -f perm_demo.txt
rm: cannot remove ‘perm_demo.txt’: Operation not permitted
[donnie@localhost ~]$ sudo rm -f perm_demo.txt
rm: cannot remove ‘perm_demo.txt’: Operation not permitted
[donnie@localhost ~]$ sudo rm -f perm_demo.txt

There are a few more commands that I could try, but you get the idea. To remove the i attribute, use the following code:

[donnie@localhost ~]$ sudo chattr -i perm_demo.txt
[donnie@localhost ~]$ lsattr perm_demo.txt
---------------- perm_demo.txt
[donnie@localhost ~]$