code:

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 This shoulddo the trick and only allow users to submit filenames with certain extensions:


<script language="javascript">
var AllowExtensions = new Array("jpg","gif","png")
var FileOK = false;

function checkExtension() {
    //we assume that Extension is false;
    var ExtensionOK = false;

    // retreive the value of the file-box
    var fieldVal = document.forms[0].elements.pic.value;

    // loop through array of allowed extensions
    for(var i = 0; i < AllowExtensions.length; i++) {
         if(AllowExtensions == fieldVal.substring(fieldVal.length - AllowExtensions.length, fieldVal.length)) {
              // The choosen extension matches one of the allowed ones
              ExtensionOK = true;
         }
    }

    if(ExtensionOK == false) {
         // file extension didn't match any of the
         // allowed ones, signal user with an alert
         // unfortunately we're not able to reset
         // the file-box
         alert(fieldVal + " is not an allowed filetype.");
    }

    // update the valid marker for the current file
    FileOK = ExtensionOK;
}
</script>

<form name="testForm" action="<% = Request.ServerVariables("PATH_INFO") %>" method="get" onsubmit="return FileOK;">
    <input type="file" name="pic" accept="image/gif" onchange="checkExtension();"><br>
    <input type="reset">
    <input type="submit">
</form> 

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