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wiki.debian.org/MimeTypesSupport has some info on mimetypes. Do you have all the .mime, .mimetype and .desktop files set up and installed correctly for the package? Also, does sly.png/svg exist somewhere in the icon search path?
This question appears to be abandoned. Unaccepted answer or unanswered, could you perhaps add more detail to your question? If this question no longer applies then you can either delete it or answer it yourself if you've solved the problem. Flagged for deletion. Thanks!