How to identify images detected before uploading is a valid method_Form Effects

Look at this!
If the image data is correct, you can get the height and width
But if you randomly select other non-web image files, you can’t get the height and width
You need to determine whether the current selected file is an image. ?

<form action="?" method="post" enctype="" multipart> <input type="file" onpropertychange="document.all.aaaa.src=this.value"> Width:<input name="W"> Height:<input name="H"> <input type="submit" value="提交"> <img name="aaaa" onload="{document.all.W.value=this.width;document.all.H.value=this.height;}" alt="How to identify images detected before uploading is a valid method_Form Effects" > </form>
[Ctrl A select all Note: If you need to introduce external Js, you need to refresh to execute ]