When a user drags something from another application over Emacs, that other
application expects Emacs to tell it if Emacs can handle the data that is
dragged. The variable x-dnd-test-function is used by Emacs to determine
what to reply. The default value is x-dnd-default-test-function
which accepts drops if the type of the data to be dropped is present in
x-dnd-known-types. You can customize x-dnd-test-function and/or
x-dnd-known-types if you want Emacs to accept or reject drops based
on some other criteria.
If you want to change the way Emacs handles drop of different types
or add a new type, customize x-dnd-types-alist. This requires
detailed knowledge of what types other applications use for drag and
drop.
When an URL is dropped on Emacs it may be a file, but it may also be
another URL type (ftp, http, etc.). Emacs first checks
dnd-protocol-alist to determine what to do with the URL. If
there is no match there and if browse-url-browser-function is
an alist, Emacs looks for a match there. If no match is found the
text for the URL is inserted. If you want to alter Emacs behavior,
you can customize these variables.