The easiest and recommended way to publish something is to upload it to your Dropbox, and then use the new “Copy Link” button to copy the link to the clipboard: It is the publisher responsibility to keep the content accessible through the link. Important thing to remember: the server does not store the actual content, only links to it. Links that fail download many times (for example because the item is no longer accessible through the published link) will be downgraded and eventually removed. The system ranks the items by download frequency, and lists most downloaded items first. Everyone can see this list and download anything from it by clicking a button. Once this is done, the link will start appearing in the list of community items in the game. Then he publishes the link to the content using this dialog in the game: Someone creates custom content (block textures and worlds are supported) and uploads it somewhere on the internet, for example to his Dropbox. This will be soon released along with 1.20, and hopefully will allow pretty much everyone to easily download custom content.
I have finished work on allowing you to share block textures and maps.