This extension includes a tweak to a standard File Manager. As you may know it does not remember the last directory when you browse images. You have to go to a needed directory every time when you want to assign an image to the product/category. This extension fixes that behavior. The last directory is preserved so you can add multiple images faster from nested directories.
It is adapted to Opencart 2.3.0.x but if you need it for older versions, please purchase the extension and contact me at email@example.com. I will adapt it for your Opencart version for free.