You can't create something from nothing. The file doesn't exist in Composer's mind until you've saved it. On the Mac you may have to use an FTP client such as Fetch to upload your files. Save yourself some headaches and be consistant when naming your files. Keep in mind that some servers including our own are case sensitive, meaning capitalization and lower case letters do make a difference.
File extensions Be Sure to include file extensions.
File extensions are the characters after the last period in a file name. Your published files should have the correct extensions for their types. It is best to use consistency when giving extensions. Multimedia files Publishing multimedia files to the academic site is okay. BUT the server is not configured to handle large streaming multimedia files, so the end result may be a frustratingly slow experience for people who are browsing to your site.