Yes. But where you ask? Within the Sandbox interface, you can click the Attachments Tab (located under the content item title) and either drag/drop a file into the dotted area, or click 'choose file' to browse your file system for the appropriate document.
If you click the Pencil icon after attaching a file an overlay will pop giving the ability to add additional meta data to the attachment.
For more details regarding our attachments feature check out our deep dive article.