How to enable a field in contact form 7 when selecting a particular value in droplist

I have created a contact form in my wordpress website using the plugin contact form7. Now I want to enable a field in the contact form only when the user select a particular value in dropdown list [in the same form].

Example: Show the form upload field when the user select the option image in the dropdown list

Please help me to do it

Thanks in adcance

via Chebli Mohamed