Let's try to figure out why we're having an issue with a temp csv file

src/PhpSpreadsheet/Reader/Csv.php

@@ -522,6 +522,7 @@ class Csv extends BaseReader
 
         // Attempt to guess mimetype
         $type = mime_content_type($pFilename);
+        var_dump($type);
         $supportedTypes = [
             'text/csv',
             'text/plain',

tests/PhpSpreadsheetTests/Writer/Csv/CsvEnclosureTest.php

@@ -153,6 +153,7 @@ class CsvEnclosureTest extends Functional\AbstractFunctional
         $writer->save($filename);
         $filedata = file_get_contents($filename);
         $filedata = preg_replace('/\\r/', '', $filedata);
+        var_dump($filedata);
         $reader = new CsvReader();
         $reader->setDelimiter($delimiter);
         $reader->setEnclosure($enclosure);