Drop zone not enabled on IE 11 (due to input.files readonly)

[FGIKCM]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• The issue still exists against the latest master branch of bootstrap-fileinput.
• This is not an usage question. I confirm having read the plugin documentation and demos.
• This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• I have attempted to find the simplest possible steps to reproduce the issue.
• I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

https://jsbin.com/zidaxis/edit?html,js,output

Expected behavior and actual behavior

With no modification of my code, this library stopped working with latest IE 11: the drop zone is now hidden, only the submit file part is visible.

After digging, it is because of:

input.files = null;

in canAssignFilesToInput() function.

An error is thrown that we can't modify read-only properties in strict mode.

Environment

Browsers

• Google Chrome
• Mozilla Firefox
• Internet Explorer
• Safari

Operating System

• Windows
• Mac OS X
• Linux
• Mobile

Libraries

• jQuery version: 3.2.1
• bootstrap-fileinput version: 4.4.8 + @dev

Isolating the problem

• This bug happens on the plugin demos page
• The bug happens consistently across all tested browsers
• This bug happens when using bootstrap-fileinput without other plugins
• I can reproduce this bug in a jsbin

[kartik-v]

Fixed via latest commit.