Information
This policy setting controls how Excel loads file types that do not match their extension. Excel can load files with extensions that do not match the files' type. For example, if a comma-separated values (CSV) file named example.csv is renamed example.xls (or any other file extension supported by Excel 2003 and earlier only), Excel can properly load it as a CSV file. The recommended state for this setting is: Enable d. (Always match file type) Excel can load files with extensions that do not match the files' type. For example, if a comma-separated values (CSV) file named example.csv is renamed example.xls, Excel can properly load it as a CSV file. Some attacks target specific file formats. If Excel is allowed to load files with extensions that do not match their file types, a malicious person can deceive users into loading dangerous files that have incorrect extensions. By default, if users attempt to open files with the wrong extension, Excel opens the file and displays a warning that the file type is not what Excel expected.
Solution
To implement the recommended configuration state, set the following Group Policy setting to Enabled. User Configuration\Administrative Templates\Microsoft Excel 2013\Excel Options\Security\Force File Extension to Match File Type Impact: Earlier versions of Excel did not enforce file type matching. Enabling this setting and selecting Always match file type might cause disruptions for users who rely on the functionality of earlier versions of Excel, and could interfere with the operation of tools and scripts that rely on it.