I'm working on a Custom webpart and would like to show icons for the extensions. This is on a Sharepoint Designer 2010. Also if i have extensions like .wav how can i show or add a custome icon. Thanks in advance
You can change File association icons globally by following steps described here:
Basic steps are:
If it's custom code then you can use SPUtility.MapToIcon to get the name of the icon in "/_layouts/images/"
If it's a view then just add the column DocIcon
For adding new images see the answer by @Vedran