Fix SpreadsheetML (xml) detection (#1917)

* Fix SpreadsheetML (xml) detection (#1916)

Replace the unrequired product signature by the
required namespace definition for XML Spreadsheet.

* Add summary to changelog (#1916)

Co-authored-by: Christof Bachmann <c.bachmann@pro54.com>

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Fix for [Issue #1887](https://github.com/PHPOffice/PhpSpreadsheet/issues/1887) - Lose Track of Selected Cells After Save
 - Fixed issue with Xlsx@listWorksheetInfo not returning any data
 - Fixed invalid arguments triggering mb_substr() error in LEFT(), MID() and RIGHT() text functions. [Issue #640](https://github.com/PHPOffice/PhpSpreadsheet/issues/640)
+- Fix for [Issue #1916](https://github.com/PHPOffice/PhpSpreadsheet/issues/1916) - Invalid signature check for XML files
 
 ## 1.17.1 - 2021-03-01
 

src/PhpSpreadsheet/Reader/Xml.php

@@ -116,7 +116,7 @@ class Xml extends BaseReader
 
         $signature = [
             '<?xml version="1.0"',
-            '<?mso-application progid="Excel.Sheet"?>',
+            'xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet',
         ];
 
         // Open file