Now you can define a header template target for modal, this is useful if you want to place a customized html in header. for this you should replace data-header with data-header-template attribute and the value should be a css selector like '#idOfDomElementsThatContainsHtml' or any css selector.
example:
<button class="lazyload showModal" data-url="@Url.Action("PreviewProduct", new {id=Model.product.Id})"
data-header-template="#productModalHeaderTemplate" data-size="modal-lg" data-callback="initProducts()">Preview</button>