Thank you for your comprehensive guide to ThingSpeak. I know some App Inventor Users, especially students, are using it (many use it with their microcontroller data). If you don't mind, it would be good to reference your post in Abraham Getzler's App Inventor FAQ. If that is OK, please give your permission here.
Concerning the battle to upload a file, a Project that includes an extension is banned by Google (classed as dangerous). MIT bans APK files as these could be malicious, or silently replaced with a malicious file.