When creating a new topic, replying to an existing topic, or sending a private message you can add pdf documents or media files (jpg plus other common photo files). To add two or more attachments to one post, you need to select them together, so they will need to be in the same folder. If you add another attachment afterwards, it just overwrites the first attachment.
You can’t upload word, spreadsheet (you will need to save these a pdf files first) or video files (use Youtube plus insert a link to it in your post).
Maximum upload size is 16MB.