Try running file appname-1.0.0.apk
I think the file command is not clever enough to identify an APK more specifically than seeing that it is a JAR, so it should print something like this:
appname-1.0.0.apk: Java archive data (JAR)
Then run jar tf appname-1.0.0.apk | grep AndroidManifest.xml
If it prints an error or nothing then there is no AndroidManifest.xml in the file.
If you do see an AndroidManifest.xml file then extract it and have a look at the contents. In an APK the AndroidManifest.xml will not actually be XML, but rather a binary format.
According to an article I found, .aab files also contain an AndroidManifest.xml, but in the .aab file it is a real XML file.
The above should at least give you a hint about what might be the problem.
One possible solution: It is possible to convert a .aab file to a .apk: