javascript alert message shown whenever I pressed enter button on all the textbox control

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ASP.NET

Javascript

Hi,

In my webpage I am used Fileupload control and upload button also. I used javascript for validate selected file is image format or not. Whenever I press enter on the textbox control the alert popup is opened. But my requirement javascript function called only when I click upload button.

My javascript code

function validateFileUpload(obj) {
  

    var fileName = new String();
    var fileExtension = new String();

    // store the file name into the variable  
    fileName = obj.value;

    // extract and store the file extension into another variable  
    fileExtension = fileName.substr(fileName.length - 3, 3);

    // array of allowed file type extensions  
    var validFileExtensions = new Array("jpg", "png", "gif", "jpeg", "tif", "tiff", "ico", "bmp");

    var flag = false;

    // loop over the valid file extensions to compare them with uploaded file  
    for (var index = 0; index < validFileExtensions.length; index++) {
        if (fileExtension.toLowerCase() == validFileExtensions[index].toString().toLowerCase()) {
            flag = true;
        }
    }

    // display the alert message box according to the flag value  
    if (flag == false) {
        alert('Files with extension ".' + fileExtension.toUpperCase() + '" are not allowed.\n\nYou can upload the files with following extensions only:\n jpg, jpeg, bmp, ico, tif, tiff, png, gif\n');
        return false;
    }
    else {
        return true;
    }
}