添加了临时目录，重启IIS服务器后终于解决了。

;;;;;;;;;;;;;;;;
; File Uploads ;
;;;;;;;;;;;;;;;;

; Whether to allow HTTP file uploads.
; http://php.net/file-uploads
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 =C:\phpfileuploadtmp

; Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
upload_max_filesize = 24M

; Maximum number of files that can be uploaded via a single request
max_file_uploads = 20