Put in some random text and click Save - you correctly change to the form action page (www.sitedoesnotexist.co.uk, which does not exist)
NOTE: This form field does not a NAME attribute and is not good HTML as the server has no way of identifying it.
Put in some random text and click Save - absolutely nothing happens !
NOTE: This form field is correctly specified; but the problem is that the non-existent file cannot have its content uploaded in the form data.
The only difference between the working and non-working, is that the mandatory NAME attribute has been included on the non working file input field.
Both browsers are trying to cope with an incorrectly specified file.
IE solves the problem by preventing form submission. This approach is reasonably sensible, however it would be much better if IE popped up a warning dialog saying "File not found". The main problem is caused by my hidden form elements which I'm using to preserve state, hence my Cancel buttons have to be INPUT TYPE="SUBMIT". Unfortunately the IE behaviour means that if an invalid filename is entered, then my users can't press Cancel to get away from the page!!!!!!!!
Firefox takes a totally different approach; it lets the server sort out an appropriate response. It means I have to code the server for this use case (good practice anyhow) and I can put up my own warning messages.
In summary, the Firefox approach needs a bit more coding but gives me complete control over the page. IE means well, but leaves my users confused and unable to leave the page.