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Why does my form with a image as the submit button does not work?
Using an image as a submit button PHP sends the coordinates of the mouse over the image upon clicking, so instead of the original submit button name (i.e. 'submitButton') two different values are send to FormEMailer: 'submitButton_x' and 'submitButton_y' - enter one of them in the submitbutton field and FormEMailer will work again.

I only see a gray box in layout mode, nothing happens in preview!
The object you see in GoLive contains PHP code which is executed on a server only. That said, you can only setup the behaviour of FormEMailer in GoLive and then have to copy, test and use this object on the server side.

I see no process indicator for the file upload!
It is not technically possible for PHP to achieve any visual feedback.

It takes a long time to process file uploads on my server!
The PHP code processes the uploaded data very fast. What you probably observe is not a delay in the PHP code but the file upload itself: upon submitting a form with a file upload, the browser sends the whole file to the server which can take some time.