Important: #106240 - Enforce File Extension and MIME-Type Consistency in File Abstraction Layer 

Key enhancements 

• Only explicitly allowed file extensions are accepted. These must be configured under the following sub-properties in $GLOBALS['TYPO3_CONF_VARS']['SYS'] : textfile_ext, mediafile_ext, or miscfile_ext.
• Files are only accepted if their MIME type matches the expected file extension. The MIME type is determined based on the actual file content. For example, uploading a real PNG image with the filename image.exe will be rejected, because image/png is not a valid MIME type for the exe extension.

New Configuration Property in $GLOBALS['TYPO3_CONF_VARS']['SYS'] 

A new configuration property, miscfile_ext, has been introduced. It allows specifying file extensions that don't belong to either textfile_ext or mediafile_ext, such as zip or xz.

New Feature Flags 

• security.system.enforceAllowedFileExtensions: Controls whether only the configured file extensions are permitted. - Disabled by default in existing installations. - Enabled by default in new installations.
• security.system.enforceFileExtensionMimeTypeConsistency: Controls whether the MIME type and file extension consistency check is enforced.

Exemptions 

Some use cases—such as importing files through internal low-level system components—may require temporary exemptions from the above restrictions.

The following example shows how to define a one-time exemption for a known and controlled operation:

<?php
class ImportCommand
{
    use \TYPO3\CMS\Core\Resource\ResourceInstructionTrait;

    protected function execute(): void
    {
        // ...

        // Skip the consistency check once for the specified storage, source, and target
        $this->skipResourceConsistencyCheckForCommands($storage, $temporaryFileName, $targetFileName);

        /** @var \TYPO3\CMS\Core\Resource\File $file */
        $file = $storage->addFile($temporaryFileName, $targetFolder, $targetFileName);
    }
}