To add media to a category, first select the category that you'd like to add media to:
Click on the ‘Add Media’ button:
Choose the file/media you wish to add either via the ‘picker’ or with a drag and drop action. In this section, you may create a name for the media file as well as upload a thumbnail image (Media Manager will create one if you do not).
You may also select the language(s) the file appears in, and select other categories you would like to add to it.
Once you have organized how and where the media should appear, click ‘Upload’.
After the upload completes, the media will appear in the category you selected. You can edit the file by clicking on the corresponding "Edit" button on the category content page.
(Note: When you are finished editing the media file, you will want to make sure and click the "Save" button.)