Fix server-side export tests (#168)

Author: MaximePinot

composer.json

@@ -37,14 +37,15 @@
         "doctrine/persistence": "^1.3.4",
         "friendsofphp/php-cs-fixer": "^2.7",
         "mongodb/mongodb": "^1.2",
-        "phpoffice/phpspreadsheet": "^1.6",
         "ocramius/package-versions": "1.4.*",
+        "phpoffice/phpspreadsheet": "^1.6",
         "phpunit/phpunit": "^8.5|^9.0",
         "ruflin/elastica": "^6.0|^7.0",
         "symfony/browser-kit": "^4.2|^5.0",
         "symfony/css-selector": "^4.2|^5.0",
         "symfony/dom-crawler": "^4.2|^5.0",
         "symfony/intl": "^4.2|^5.0",
+        "symfony/mime": "^5.1",
         "symfony/phpunit-bridge": "^4.2|^5.0",
         "symfony/twig-bundle": "^4.2|^5.0",
         "symfony/var-dumper": "^4.2|^5.0",