It is very simple to configure a custom uploader for Hubzilla:
1. Press the cog button 2. Press the + button to add a new uploader 3. Select POST as method 4. Enter the upload URL https://channelname:password@hub.example.com/api/z/1.0/item/update 5. Set Form Data Name to media
Done.
Sharing a file with this uploader will result in a post to your default privacy group with the file attached or embeded if it's an image.
You could also add a custom text to your upload if you add the body upload parameter or even configure a privacy group to post to if you want.