We had the same problem with our Forma installation (2.3). Then, we realized that the problem disappears when we switch from one PHP version to another one.
We used PHP 5.6 to install FormaLMS because it was not possible to use PHP 7.1 during installation. After completing the installation, we switched to PHP 7.1 and FormaLMS still worked. However, we had a blank window for upload (and for the media folder altogether). Switching back to PHP 5.6 solved the problem. As your PHP version is 7.0, I suggest you try this solution. I hope it will work for you, too.
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