Lets start here, first MP4s aren't actually image files. They are video files. So they would never be viewable as image files.
Now in regard to accessing the phone from other devices, it depends on how you are doing it.
First, try restarting the phone (just turn it off and back on). Then connect it to the computer via USB. You should be able to, from the computer, open up the "Internal" storage or the "External" storage (SD card) from the computer by opening it up as a folder. You don't need to use the manufacturers software. Just open it up as a folder like you would for any location on the computer itself. From there you should be able to move files back and forth from one device to the other.
When the system on the phone updated, was it an update that came through the phone's "Settings" or is this something you chose to install? If the latter, then it was not an update, and that could be your problem. If that is the case, I would uninstall it. If, however, it is the former, then you may need to see if any of the apps you have on the device are hindering what you are trying to do. Some have conflicts with phones after updates.