For (at least) gallery avatars.

Having an avatar file named

1!@#$%^()-_=+.jpg

in the folder

a!@#$%^()-_=+

Can see and select the avatar in the UCP. URL in UCP is:

a%21%40%23%24%25%5E%28%29-_%3D%2B/1%21%40%23%24%25%5E%28%29-_%3D%2B.jpg

After selecting, the avatar does not display for the user's profile. URL is:

a!@#$%^()-_=+/1%21%40%23%24%25%5E%28%29-_%3D%2B.jpg