v2: FileUpload does not respect `setMultiple(false)`

[FrankHossfeld]

Using the FileUpload element and setting setMiultiple(false) does not work as expected. Instead of only uploading one file Item, it is possible to upload more than one item.

In v1 it works as expected.

In case that setMultiple(true) is used and another file is selected, the existing uploaded item has to been removed and the new one loaded. In case of drag & drop of several items, only one file should be processed and the others items should be ignored.