如何限制客户上传的文件类型？只能让客户上传某一类型的文件

如题。
如何限制客户上传的文件类型？只能让客户上传某一类型的文件

1<input name="file" type="file"/>

最好载这个页面就判断
---------------------------------------------------------------

1@LANGUAGE="VBSCRIPT" CODEPAGE="936"

 1   
 2set upload=new upload_5xsoft   
 3set file=upload.file("file")   
 4  
 5str=file.filename   
 6txt=split(str, ".")   
 7if (txt(1)<>"html" and txt(1)<>"htm") then call error() end if   
 8if (txt(1)="html" or txt(1)="htm") then   
 9.   
10.   
11.   
12.   
13end if   
14  
15  
16sub Error()   
17response.write "

<br/>

1"   
2response.write "

<table align="center" border="1" bordercolor="#999999" cellpadding="0" cellspacing="0" width="300">"
response.write " <tr bgcolor="#6699CC"> "
response.write " <td colspan="2" height="15"> "
response.write " <div align="center"><font color="#FFFFFF">你所选择的文件类型不正确</font></div>"
response.write " </td>"
response.write " </tr>"
response.write " <tr> "
response.write " <td colspan="2" height="23"> "
response.write " <div align="center"><br/><br/>"
response.write " 请选择html或htm文件<br/><br/>"
response.write " <a href="javascript:onclick=history.go(-1)">返回</a>"
response.write " <br/><br/></div></td>"
response.write " </tr> </table>

1"   
2end sub