是否每个文件都有一个关联的MIME类型

我最近实现了一个安全系统，在该系统中，我们根据可接受的MIME类型和文件扩展名列表进行检查。如果扫描的文件在此列表中具有MIME和扩展名，我们将继续前进。我已经包含了我们扫描下面文件的功能。ALLOWED_EXTENSIONS和ALLOWED_MIME_TYPES只是字符串，如"txt，pdf，jpeg…."。

我想你知道MIME类型是什么以及如何工作的，但最近我们上传的PDF根本没有MIME类型。顺便说一下，这段代码大部分时间都能正常工作。我见过PDF经过精细处理，还有图像、文本文件等。

一个文件可能根本没有MIME类型吗？

 /**
 * scan the file before upload to do our various security checks
 *
 * @param  tmpName    the file's location in /tmp, used for MIME type scan
 * @param  name       the filename as it was uploaded, used for extension scan
 * @param  oid        the order id, passed along to notifyStaffIllegalFileUpload() if email needs to be sent
 * @return            true on success, error string on failure
 */
function scanFile($tmpName, $name, $oid) {
    global $_email;
    // get lists from config
    $allowedExtensions = explode(",", ALLOWED_EXTENSIONS);
    $allowedMIMEs = explode(",", ALLOWED_MIME_TYPES);
    // get extension
    $ext = pathinfo($name, PATHINFO_EXTENSION);
    // get MIME type
    $finfo = finfo_open(FILEINFO_MIME_TYPE);
    $mime = finfo_file($finfo, $tmpName);
    finfo_close($finfo);
    // check against allowed
    if (!in_array(strtolower($ext), $allowedExtensions) || !in_array(strtolower($mime), $allowedMIMEs)) {
        capDebug(__FILE__, __LINE__, "Order #" . $oid . " - A user attempted to upload a file with extension '" . $ext . "' and MIME type '" . $mime . "'. The attempt was blocked.'n", "/tmp/file_errors.log");
        $_email->notifyStaffIllegalFileUpload($oid, $name, $ext, $mime);
        return "Our security systems detected an illegal file type/mime type. The file upload was cancelled.";
    }
    return true;
}