You may not be able to view a file because some file formats aren't supported, and those files can't be viewed. For example, a .zip file can't be viewed but if you have adequate permissions (at least the Downloader role), you can download the file for use.
Your service administrator may also have prohibited certain file types, so you won't be able to view those.
File previews are limited to the first 100 pages. The entire file size must be less than 158MB.
Supported video file formats are dependent on your browser, which must support the HTML5 video tag. The currently supported formats are:
Internet Explorer: MP4
Chrome: MP4, WebM, and Ogg
Firefox: MP4, WebM, and Ogg