Also, I found in the Appplication/Xcode/..../DeviceSupport, the iOS SDKs are only up to 15.2
greenriverpark
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My bad, a typo, 1. I CAN define my application defined type files like .AAA and .BBB
I still don't know how directly solve this issue, but found a work-around, basically, add addition file type like, .CCC, and combine. .AAA.CCC. and .BBB.CCC to represent type A and type B files that are defined by my application. when sending a mix of type A and type B files, the files are like. (.AAA.CCC , .BBB.CCC, .AAA.CCC and etc.) and in my app, when importing files, will treat. .AAA.CCC and .BBB.CCC files differently.