Prevent browser from loading a drag-and-dropped file

After a lot of fiddling around, I found this to be the stablest solution:

var dropzoneId = "dropzone";

window.addEventListener("dragenter", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
}, false);

window.addEventListener("dragover", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
});

window.addEventListener("drop", function(e) {
  if (e.target.id != dropzoneId) {
    e.preventDefault();
    e.dataTransfer.effectAllowed = "none";
    e.dataTransfer.dropEffect = "none";
  }
});

<div id="dropzone">...</div>

Setting both effectAllow and dropEffect unconditionally on the window causes my drop zone not to accept any d-n-d any longer, regardless whether the properties are set new or not.

You can add a event listener to the window that calls preventDefault() on all dragover and drop events.
Example:

window.addEventListener("dragover",function(e){
  e = e || event;
  e.preventDefault();
},false);
window.addEventListener("drop",function(e){
  e = e || event;
  e.preventDefault();
},false);

For jQuery the correct answer will be:

$(document).on({
    dragover: function() {
        return false;
    },
    drop: function() {
        return false;
    }
});

Here return false will behave as event.preventDefault() and event.stopPropagation().