Castmill support most media types out there for images, videos, pdfs and audio files. All medias are handled in the Medias tab, where you can upload, preview and organize them.
The first time you enter Castmill it will appear empty until you start uploading content:
The medias page
Just click on "Update Medias" to upload the content you want. A dialog will open and you can choose as many media files as you want in one go:
Click OK and now you can see the list of files ready to upload:
The files will upload as soon as you click the "Upload" button: