Completed onChange will reset FileInput value to null

When attaching a file using the "Select File" button, it was not possible to attach the same file after deleting it. Re-attaching the file worked as expected when using Drag and Drop. Investigation showed that the onchange event occurred only when the value of an element was changed, so in this case trying to attach the same file did not change the value of the element and did not fire the onchange. To resolve this, the value of FileInput in pzMultiFilePath Control will be set to null at the end of the onChange event so that an onChange event will be triggered next time even if the same file is added.