Currently, Spark does not support directly uploading attachments to emails, but you are able to upload your content to Spark and then include a link to it in your message.
To do this, upload your file in the Files section on your Spark project. You'll notice that each row in the list has three grey dots on the right side - clicking this will open up a micromenu, where you can select the "Copy" option. This will copy the link to your clipboard.
Return to your template or email and paste the link in your desired format.