I am new on iOS but finally found a way to fix stuck Preflighting Application issue on Cydia Impactor.
Need basic hacking skills ;)
You should modify target ipa's "Info.plist" file and change needed minimum iOS version 1.0 (from 8.4.1) for example. For this;
- open ipa file with winrar or any rar extractor
- find Info.plist file and extract it
- use any hexeditor (preferred xvi32) and open Info.plist
- search (with your eyes) any minumum iOS version parameter (for example 8.4.1 etc...)
- edit it with 1.0 (for example; modify 8 with 1, modify 4 with 0 and "MODIFY . AND 0 (2E AND 30) WITH 00 00 HEXES!"
- copy Info.plist back to rar archive and select "store" optin from compression level
- use cydia impactor and sideload ipa