原因：文件超过限定大小或没指定临时目录
修改php.ini配置
file_uploads = On
; Temporary directory for HTTP uploaded files (will use system default if not
; specified).
; http://php.net/upload-tmp-dir
upload_tmp_dir ="/tmp"
; Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
upload_max_filesize = 8M