To preview you need to select the file from the downld window & press the preview button. If the file is still in the download window then it's not completed. Sizes can be a little confusing because not all parts of files are downlded sequentially. So you may have most of the 38% but also a part of the end of the file. So your system thinks the file size is larger than it is. That space is reserved until the downld completes. This often happens with video files. Particularly mpg files.