After enough tinkering I managed to come up with a more suitable solution...
Seeing I'm only really focusing on the prevention of files being dropped into sortable area, I decided to focus on determining the nature of the item being dropped...
drop = $(".all-slides");
drop.on('dragover', function(e){
e.preventDefault();
});
drop.on('dragenter', function(e){
e.preventDefault();
});
drop.on('drop', function(e){
e.preventDefault();
dt = e.originalEvent.dataTransfer;
if(dt.types != null && (dt.types.indexOf ? dt.types.indexOf('Files') != -1 : dt.types.contains('application/x-moz-file'))) {
alert('You cannot upload images here. \nIf you wish to upload files, \nclick on the "Upload Images" button.');
}
});
Put simply, the drop event listens for any possibility of data transfer, and if there is no data transfer possible, we know it's a DOM element... Otherwise we know it's a file, and we throw the alert...