Add Attachment: If the forum staff has attachments enabled, you will see this button on the posting page. After you click the Add Attachment button, a new window will pop up letting you know how many files you can upload at a time, along with what the file size limit per attachment is. To attach a file, click the + Add Files... button, and then choose a file that you would like to upload from the Browse window that pops up. Once you choose a file, you will see a progress bar for the attachment that is uploading.
Once the attachment has been made, you can use the Insert button to have the attachment appear inline within your post, or you can use the Delete button if you decide against adding an attachment. Otherwise, you can click the Done button found at the bottom of the pop-up window and your attachment file will appear at the very end of your post after your thread has been made.