Now I'm not sure the file type has anything to do with it. I added a new field to the popup ItemDetails form, then after copying the new SWF and hitting refresh, I get the below error. I then simply recompiled the app (without making changes) and replaced the SWF, then the error went away. This is truly sporadic.
ReferenceError: Error #1065: Variable ItemDetails__embed_mxml_assets_printer_png_1274121104 is not defined.
This is by far the biggest anomaly I've seen in Flex. The SWF throws the error, then after recompiling and using the new SWF twice (without changing the source code), it works the second time.
In my research I found this post: http://kachurovskiy.com/2010/storing-icons-in-external-zip-and-seamless-work-with-composit e-icons/
I'm not the only one who has experienced this issue - (Google "Flex ReferenceError: Error #1065 ")