1.限制文件请求类型，一些常见的文件类型

后缀	MIME
*.csv	text/csv
*.doc	application/msword
*.ppt	application/vnd.ms-powerpoint
*.jpeg	image/jpeg
*.jpg	image/jpeg
*.mp3	audio/mpeg
*.mp4	audio/mp4, video/mp4
*.pdf	application/pdf
*.png	image/png
*.txt	text/plain
*.zip	aplication/zip

2.根据文件后缀限制

#方法一:pathinfo
    $extend = pathinfo($filename);
    if($extend['extension']!='txt'){
            return false
    }
#方法二:$_FILES
    if($_FILES['file']['type']!='txt'){
            return false
    }