The File Attach option is particularly useful when dealing with large files. It allows you to upload a file through other means (such as FTP) and then reference that file through the file field. The reason this is so useful is because many php installations limit the file size that can be uploaded through the browser as well as the length of time the system should process an upload. Meaning, if it is too large, and takes too long, the system will time out, and the file will not get uploaded. In some instances, you can increase these limits to a point, but many hosts do not allow you to exceed a specified limit. So it's useful to have another option.