I have the same issue. I updated my iMac to 10.3 and now I can’t upload. My older iMac still works because I’m still on the older version.
I hope they fix this bug.
I am also facing the problem while uploading file.
I am using blueimp file uploader it is working in older versions of safari and all other browsers (chrome and firefox)
but not working in safari 11.1
Plz help