file_file_download should delegate header checks to a separate function

There is some lovely code in file_file_download that determines what a file object's headers should be on download. If it's an image or text, it sets the content-disposition to inline; if it's a PDF or something, it sets it to force a download. It also sets the content type and length correctly. Unfortunately, all this lovely code is buried inline, inside of file_file_download(), where contrib can't reuse it. Patch forthcoming.