File by OS
File by OS can differ in both format and function. For example, a primary executable format in Microsoft Windows is an EXE file, but Linux has its own executable format, which also has no file extension! Another example is command files, which often have a "bat" extension for Windows and "sh" in Linux.
Multi-Platform File Formats
To avoid problems in file conversion and exchange, some institutes are creating standards for representing different information types for any OS that you can use. Most formats for modern files are cross-platform, allowing using files in these formats under any modern operating system.
Note: If you can convert your particular data to a multi-platform file format when creating it, do it! If you still have a deed to convert your files to use under other OS and its unique software, please stick to file converter utilities!