Your company has evidence of storage of adult films in the computer that is allotted to you for the work. Now you are on weaker side. Therefore, at this stage, I feel the only way out for you is to accept the misconduct and ask for apology.
In the reply you may write that though you had copied the adult film material inadvertently on your PC/laptop, you did not further transmit or disseminate this material further to anyone.
Your only strength today is your 18-year long service. Hopefully your company takes into consideration your long tenure in the company and do not award to you very severe punishment like termination.