Proper mime type detection for XLSX files

Heuristics to detect proper mime type for XLSX files expect to see
certain files at the beginning of the XLSX archive. The order in which
the XML files are added therefore matters.
Specifically, "[Content_Types].xml" should be added first, followed by the
files located in the "xl" folder (at least 1 file).

tests/Spout/Writer/XLSX/WriterTest.php

@@ -385,6 +385,21 @@ class WriterTest extends \PHPUnit_Framework_TestCase
         $this->assertInlineDataWasWrittenToSheet($fileName, 1, 'control's _x0015_ "character"');
     }
 
+    /**
+     * @return void
+     */
+    public function testGeneratedFileShouldHaveTheCorrectMimeType()
+    {
+        $fileName = 'test_mime_type.xlsx';
+        $resourcePath = $this->getGeneratedResourcePath($fileName);
+        $dataRows = [['foo']];
+
+        $this->writeToXLSXFile($dataRows, $fileName);
+
+        $finfo = new \finfo(FILEINFO_MIME_TYPE);
+        $this->assertEquals('application/vnd.openxmlformats-officedocument.spreadsheetml.sheet', $finfo->file($resourcePath));
+    }
+
     /**
      * @param array $allRows
      * @param string $fileName