Uploading a file is easy, but it does require you to have Pro. Once you have pro if you want to attach audio, video, or even other file types like PDF content you can do so after your conversation is made.
Once the conversation has been made you can click the 3 dots in the top right hand side and a menu will appear with options including Attach File. Once you attach a file it will show up for users to see, it will render a thumbnail if your content is a video.
You can also attach photos that will be formatted and rendered in so no warning is shown to view the image.
A warning will render on any attachment you upload before a user is allowed to open it for security reasons.