The Media Manager file uploader allows for the ability to upload multiple files at one time.
From the side menu, select Media Library
When the page loads, select Add Media, found on the top left
From here, you will be able to choose a category for the media to be placed in and a file from which you want to get media from.
When choosing a category, a new window will open. This window contains all of the groups that you have created. When you choose a group, the corresponding categories will appear. Select as many groups and categories at one time as needed.
When you are ready to choose files, drag and drop or press "Choose Files" to open a file select box. Select as many files as desired and click open.
You will now see the list of media files in the upload box. From here, you can add more content or remove unwanted content simply by clicking the X on the left side. When you are ready, press the red upload button.
After uploading, the manage content page will open and your media within the category (or one of the categories you chose) will show!