Preventing replacement of a web page by something dropped on it

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Default behaviour for a browser displaying a page, when e.g. an image
file is dropped on it, is to replace the page and display the image. Is
there a way to prevent this without using javascript?

With javascript, I can set event handlers for dragover and drop, in
which the drop event is essentially ignored, thus preventing the
symptoms. However in once instance I have a sandboxed iframe
(specifically to prevent javascript execution) into which I put
arbitrary, untrusted, html. A way to prevent the replacement of this
content by a dropped image would be welcome.

(I hope I'm overlooking something obvious :-)

"That excessive bail ought not to be required, nor excessive fines imposed,
nor cruel and unusual punishments inflicted"  --  Bill of Rights 1689

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