Open a web page with a file input element which includes the accept attribute with a value of "image/*" When you click on the "Choose File" button Safari will show the open file panel, allowing the user to only choose image files. But you can drag a .pdf file over the file input and it will accept it.
<rdar://problem/10415311>
It could be a weird user experience if a drag is accepted, but some files in a drag are ignored. Do any other browsers do this?