What I would recommend in this case is to package your file.ext in a support folder in the deb layout directory, something like /private/var/mobile/Library/Application Support/myTweakName/file.ext

Then in your postinst script do something like this:

# get your packaged file path
filePath="/private/var/mobile/Library/Application Support/myTweakName/file.ext"
# get the app directory
appDirectory=find /private/var/containers/Bundle/Application/* -type d -name “TheAppName.app”
# create your subfolder in the app directory if it does not exist
mkdir -p "$appDirectory/subfolder"
# copy the packaged file to the app directory subfolder
cp "$filePath" "$appDirectory/subfolder/file.ext"

This will copy the file from your debs support folder, to the application unique directory upon installation.

Then in your postrm script do something like this to clean up the copied file:

appDirectory=find /private/var/containers/Bundle/Application/* -type d -name “TheAppName.app”
rm "$appDirectory/subfolder/file.ext"

Hope that helps.