JSFiddle - React, Tailwind, and code Playground

HTML

<form name="upload-form" id="upload-form" action="upload-information.php" enctype="multipart/form-data" method="post">

    <input type="file" name="dealers_csv" id="dealers_csv" required="required" data-message="Please attach the mcust.csv file.">

    <button type=\"submit\" name=\"Submit\" id=\"Submit\" class=\"csbutton-color upload-button\" style=\"margin-right:10px;\" >Submit files</button><img id=\"loader\" src=\"../images/loading.gif\" style=\"display:none;\"/>

</form>

JavaScript

$("#dealers_csv").change(function(){
        var dealers =$(this).val();
        if(!dealers.match(/.*\\mcust.csv$/g)){
            alert("Wrong file!" + dealers);
            this.value = "";
        }
     });