You can optionally attach one or more files (the number depends on the forum settings) to any post in the forums.
This can useful when you want to share a picture or a word processing document, for example.
Your files can be of any type, however it is highly recommended that the file is named using standard 3-letter internet suffixes such as .doc for a Word document, .jpg or .png for an image, and so on. This will make it easier for others to download and view your attachment in their browsers.