Override method to determine the document type from a file. If it's a type we can't open, try to check for the file header, it might be PDF or PS with the wrong extension. 
Override method to determine the document type from a file. If it's a type we can't open, try to check for the file header, it might be PDF or PS with the wrong extension. 

git-svn-id: cb3fc60f0f557b927b8032d8a94e869b6f9f230b@2522 0fceea05-a30d-0410-8a8b-80ef821fb0a1

camillobruni/skim,camillobruni/skim,camillobruni/skim,jasongrout/skim,camillobruni/skim,jasongrout/skim