I can almost guarantee that they would be using different things. usually you have simpler libraries to decode formats (almost 1 for each codec), and separate programs plug these libraries in to generate the output. previews do not have to be accurate and have to be fast, so a simpler program with just linear scaling or something, where as actual image would be complex which has to worry about accuracy.

still not a excuse to not have support for a free 15 year old format