extra.dat

Generic data file

A .dat file contains generic data. The extension does not identify one format, and each program defines its own structure. Examples on Windows are registry hive files such as NTUSER.DAT and different program caches. Some .dat files are plain text, and others are binary. To identify the content, examine the file data or the program that made the file.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.