fix uploading files via ajax in Safari 11

[Ciki]

before form submission, clone form and remove empty file inputs as these causes Safari 11 to stall

https://stackoverflow.com/questions/49672992/ajax-request-fails-when-sending-formdata-including-empty-file-input-in-safari

[PavelJurasek]

I was just investigating the issue with 503 response when submitting ajax form in Safari. I just tested it and it works like a charm 🙏

[Ciki]

hah, spent couple of hours to hunt it down and fix. glad you didn't have to :)

[Ciki]

@PavelJurasek I updated the changes as the original did not copy dynamic form values like select or textarea values due jquery.clone() implementation. it should work flawlessly now.

[vojtech-dobes]

Thanks!