Filefield or imagefield

You can't magically make a FileField and ImageField, CCK will always recognize a widget type as whatever it was original created as. However you can accomplish what you're asking simply by theming the FileField Widget to show an image if an image is uploaded. You could do similar things to make audio files have a player or video files show a thumbnail.

To do this, just override the theme_filefield_widget_preview() function, which provides the preview next to the uploaded file. Take the function out of filefield_widget.inc, copy it into your theme's template.php, and rename it to [yourtheme]_filefield_widget_preview(). Then do any changes you want, and clear your Drupal caches at admin/settings/performance.

but when uploading let js check for file extension, and if is jpg/gif/png use imagefield otherwise use filefield?

There is no easy way to accomplish this. It sounds like a difficult thing to implement that would be very sensitive to changes on the site (such as theming the node form, or positions of the FileField and ImageField). ImageField is purely built on top of FileField, so between the two of them they should have very similar functionality.

I would love to see this too.

I've long thought about having FileField detect the image types and display them with the ImageField widget.

This will not be added. FileField is essentially discontinued since it has been integrated into Drupal core.

look at FileField Styles, which does this (and works in d7 as well).