uninstallation - Uninstall system app failed using 'adb shell pm uninstall'

I attempted to force uninstall in ADB, but failed:

$ adb shell pm uninstall -k org.openintents.filemanager
$ adb shell pm list packages -f | grep -i manager
package:/system/app/FileManager.apk=org.openintents.filemanager

What went wrong here? What can I do?

I used the internal application manager and there isn't an option to delete it.