OverwriteInitial + ClearStack not working

[blacko2017]

@kartik-v Now there is a new problem. If you click Multiple upload button on a non-multiple file, the overwriteInitial does not work.

If you try getFileStack, it shows the correct list, 1 file, but in the preview there are 2 or more.

Steps to reproduce the issue

1. You have an image for initialPreview and you set overwriteInitial true;
2. You select another image, the initialPreview disappears;
3. You press Upload and the first initial preview appears again.

Expected behavior and actual behavior

Only the new uploaded image should appear, but it shows all old files, not replacing.

[blacko2017]

After changing the image the third time, it shows all old previews.

[blacko2017]

Ok I found that ClearStack removes only not-uploaded files, so it is working.

About the not-overwriting problem, I found that in my case the problem is the command in row 1663:
self.overwriteInitial = false;

It disables overwriteInitial after first upload. But in the case of a single upload, it should be ALWAYS overwrote. So it's not an issue/bug but it should be introduced in my opinion!

## Edit ##

Finally found a solution:
if(self.$element.attr('multiple')) self.overwriteInitial = false;