Only reset the image, not the complete form

[bobvandevijver]

The current code resets the complete form whenever the reset button is in the single upload widget. As the button only appears when the file is changed, I classify it as unwanted behaviour to reset the complete form when you only want to cancel the file change. To cancel the whole form you need to place a reset button at the bottom (or top, or whatever).

Simply removing the button property will result in an automatic submit (I'm not sure why), but just changing it to the type button fixes the problem.

[bobvandevijver]

@loostro Can you look at this PR? And preferrably merge it? :)

[sescandell]

Thank you @bobvandevijver