Restrict WordPress File Type Uploads by User Type

There is a syntax error in your conditional:

current_user_can(‘administrator’)

The input value is wrapped in ‘ ’, which should be wrapped in ' ' instead. Right now, because ‘administrator’ is neither a role nor capability, the above will always return a false value, therefore

if(!current_user_can(‘administrator’))

will always return true, which will restrict the mime type for everyone, including administrators. The correct form will be :

if( !current_user_can('administrator') ) { 
    //CODE HERE
}