Files upload not works in IE10 and IE11

Thank You attiks for useful module!

I use it in project and found if:

• Add to any webform Multifile item
• Open this webform in IE10 or IE11
• Attach 2 files with Multifile item
• Submit webform
• Open added submission
• Added files will be not attached to submission

This appears because:

• [] symbols was not added to Multifile form input element
• As result on webform submit _webform_validate_multifile() validation failed because $_FILES['files'] structure is not correct.

Attached patch fix this issue.