Uninstall system app failed using 'adb shell pm uninstall'
Well, since the FileManager.apk package resides in a /system/app/ directory it is a system app. So the general way to remove it is:

adb shell
su
mount -o rw,remount /system
rm -rf /system/app/FileManager.apk
rm -rf /data/data/org.openintents.filemanager
mount -o ro,remount /system
exit
exit