Hi,
If you right click the element you want to remove and select “Inspect” it opens the developer tools and you can see if it has an id or class you can use to select it. In this case the element you want to remove has the class “ms-propertysheet” and you can add this to your Custom CSS in EditForm to hide the delete button:
/* Hide delete attachment button */
.ms-formbody .ms-propertysheet{
display:none;
}
Because the class might be used in other places as well I have included the ms-formbody class in the selector to only hide ms-propertysheet when it is inside an element with class ms-formbody.
Alexander