I recently came across https://www.drupal.org/project/file_upload_secure_validator

This is a very simple and small module which performs a server side validation for the extension of an uploaded file of any content type's file field. Default drupal 7 file validation is performed by file_validate_extensions() which only relies on the file name extension.

'File Upload Secure Validator' uses the php library fileinfo and is dependent on that. Therefore the server hosting the drupal instance should have this library enabled. Through this php lib we can perform a more secure and reliable check on the file's mime type and compare that to the allowed file extensions, as these are set by the admin within the content type's field settings.

This module is useful when we need to enforce a maximum security mime type detection.

Dependencies The module depends on the php library fileinfo. Please make sure this library is present and enabled on the server.

Installation Install module like usual. No special installation considerations

Configuration No configuration options. After enabling the module, it will perform an alternative server side extension validation on every uploaded file of every content type file field.

GitHub Issue #: 
3830