With further analysis I have found I was wrong about the use of Ajax. The client side script in InputfieldFile.js was using Ajax based file upload but it was just not working and not returning any errors. As soon as I commented out the following code block and replaced it with InitOldSchool() both image and file uploads worked fine not using Ajax.
* if (window.File && window.FileList && window.FileReader && $("#PageIDIndicator").size() > 0) {
* InitHTML5();
* } else {
* InitOldSchool();
* }
So any guess as to why Ajax based file uploads have stopped working or how I can debug this?
I have tested this on both IE, Chrome and FF with no change. It must be a hosting config issue.