attachment - Show attachment as full size image
attach - Shows the attachment as a thumbnail, expandable to full size.
attachthumb - Shows NON-EXPANDABLE thumbnail of the attachment.
attachmini - Shows the attachment as a thumbnail, expandable to full size. Download count and filename omitted beneath (regardless of ILA settings).
attachurl => Shows the attachment like you used an url tag instead of this tag.
example: [ attachment=1 ] will display your first uploaded image in your post.