TYPO3 Core

file mount directory e.g. "test.com" will be classified as file, if the directory name has point.

TYPO3:
  CMS:
    Form:
      persistenceManager:
        allowedFileMounts:
          100: '1:/test.com/'

The problem is using pathinf() to check if $persistencePath is a file or a directory, in .../typo3/sysext/form/Classes/Mvc/Persistence/FormPersistenceManager.php

    public function isAllowedPersistencePath(string $persistencePath): bool
    {
        $pathinfo = PathUtility::pathinfo($persistencePath);
        $persistencePathIsFile = isset($pathinfo['extension']);
        ...