--- a/typo3/sysext/form/Classes/Slot/FilePersistenceSlot.php
+++ b/typo3/sysext/form/Classes/Slot/FilePersistenceSlot.php
@@ -140,7 +140,7 @@
         $this->assertFileName(
             self::COMMAND_FILE_ADD,
             $combinedFileIdentifier,
-            file_get_contents($sourceFilePath)
+            file_get_contents($sourceFilePath, false, null, 0, 33554432) // size of checksum hash in byte = 32 MB
         );
     }