There are many types of files stored on a computer's hard drive including operating system (OS) files (for example, Windows or Macintosh files used to handle specific tasks performed by the OS), image files, and Excel files. Most of these files are created by specific applications and therefore are proprietary. Proprietary files should only be accessed by the applications from which they were created. In Windows, proprietary files have unique file extensions such as .doc, .xls, and .ppt to name just a few. The file extensions are used by the OS to identify the application that created the file.
A proprietary file (such as one created by Excel) contains not only the textual and numerical information entered by the user, but also content that the application uses to specify formatting options selected by the user (bold, font size and type, and so on) as well as any non-textual information entered by the user (for example, images and charts). The methods used to write all of this additional information to the file are specific to the application, and therefore can only be opened by the application that originally created the file.
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