Since for a normal image one always should use thumb to scale it, I suggest to preselected "thumb" in the format dropdown and to append it in non-dialog mode. If you need some statistics: 59 % of all images in the German Wikipedia (not counting images on discussion and user pages) use "thumb" or an equivalent parameter (that's a fact) and among the other images there are many where "thumb" is just missing because the editor didn't know of it (that's my opinion).
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