that file is very "wrong"
the hole top part is inverted and is not connected to the bottom part except in few points only. this looks like a very bad executed boolean between two objects .. you can try to import into some mesh editor (art of illusion for e.g.) and manually fix this but comment in the file states it's made by openscad, you probably have a source so try a different approach in making this STL, maybe report a bug to openscad developer with what exactly did you tried to do.. maybe it's a bug in openscad ...
were you embossing image on a cube or ?
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