Hi,
To change the default setting you must go to View---Attachments---Inline in the main Kmail window, not in an open email. Changing this setting in an open email only changes the setting for that one email. When you first open up Kmail do: View---Attachments---Inline before opening up any email and it will change the default; however, you may need to restart Kmail in order for the new setting to take place.
I hope this help you.