adding ringtones

I am wanting to add personal ringtones and assign to some of my contacts. I have used a ringtone app and have downloaded a few. I go to a contact and click on the option to 'Set ringtone' which takes me to the availble ringtones that came with the phone. The last option in this list is "+ Add ringtone". I click on it and it opens the folder where I stored my downloaded ringtones. I click on the ringtone I want to add which closes the folder and takes me back to the list of available ringtones but the one I selected is not added to the list.