In this article I will explain with an example, how to filter files based on extensions in ASP.Net FileUpload control.
When a file is selected in the FileUpload control, the extension of the selected file will be validated using JavaScript and Custom Validator.
HTML Markup
The HTML Markup consists of:
FileUpload – For selecting file.
Button – For uploading file.
CustomValidator – For validating and displaying appropriate message.
The CustomValidator control has been assigned with ClientValidationFunction which is set to ValidateFile JavaScript function.
<asp:FileUpload ID="fuUpload" runat="server" />
<asp:Button ID="btnSubmit" runat="server" Text="Submit" />
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="" ForeColor="Red"
    ControlToValidate="fuUpload" ClientValidationFunction="ValidateFile" />
Validating File using CustomValidator with JavaScript
When file is selected, ValidateFile JavaScript function is called.
Inside the ValidateFile JavaScript function, the file extension of the uploaded file is determined and an Array of valid file type is defined.
Then, the file extension is verified whether exists or not in the Array using the Array.includes function and returned value is set in IsValid argument.
The following Array can be easily set with valid file extensions as per your requirement.
var validFilesTypes = ["doc", "docx", "pdf"];
Finally, if the IsValid argument is false, then error message is set in the Custom Validator.
<script type="text/javascript">
    function ValidateFile(sender, args) {
        sender.innerHTML = "";
        var filePath = document.getElementById(sender.controltovalidate).value;
        var extension = filePath.substring(filePath.lastIndexOf(".") + 1, filePath.length).toLowerCase();
        var validFilesTypes = ["doc", "docx", "pdf"];
        args.IsValid = validFilesTypes.includes(extension);
        if (!args.IsValid) {
            sender.innerHTML = "Invalid File. Please upload file with extension: " + validFilesTypes.join(", ") + ".";
Filtering Files based on Extensions in ASP.Net FileUpload Control
Browser Compatibility
The above code has been tested in the following browsers.
Internet Explorer  FireFox  Chrome  Safari  Opera
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