framework/helpers/BaseFileHelper.php
protected static function loadMimeAliases($aliasesFile)
    {
        if ($aliasesFile === null) {
            $aliasesFile = static::$mimeAliasesFile;
        }
        $aliasesFile = Yii::getAlias($aliasesFile);
        if (!isset(self::$_mimeAliases[$aliasesFile])) {
            self::$_mimeAliases[$aliasesFile] = require $aliasesFile;
        }

        return self::$_mimeAliases[$aliasesFile];
    }
protected static function loadMimeAliases($aliasesFile)
    {
        if ($aliasesFile === null) {
            $aliasesFile = static::$mimeAliasesFile;
        }
        $aliasesFile = Yii::getAlias($aliasesFile);
        if (!isset(self::$_mimeAliases[$aliasesFile])) {
            self::$_mimeAliases[$aliasesFile] = require $aliasesFile;
        }

        return self::$_mimeAliases[$aliasesFile];
    }
Loads MIME aliases from the specified file.
@param string $aliasesFile the path (or alias) of the file that contains MIME type aliases.
If this is not set, the file specified by [[mimeAliasesFile]] will be used.
@return array the mapping from file extensions to MIME types
@since 2.0.14