What I do here is to create the relevant sized icons as PNG files, I upload them into the correct directory and the I modify the CSS for the component (and potentially the plugin too!) so that, by default, the PDF icon is presented every time. Of course. If you need different icons for different file types, you'll maybe not prefer this route, but on my sites the files are invariable PDF.
A thought for Jan...
Could we maybe have a feature on the file upload panel to choose a CSS style? Then we can declare the style?