JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <form onsubmit="Validation">
    <div id="deplacemoi">J'accepte les fichiers déplacés !</div>
    </form>
</body>

CSS

div {
        margin: 50px auto;
        text-align: center;
      }

      #deplacemoi {
        padding-top: 40px;
        width: 400px;
        height: 60px;
        border: 2px solid #222;
        background-color: #888;
      }

JavaScript

var _validFileExtensions = [".jpg", ".jpeg", ".bmp", ".gif", ".png"];    
function validate(files) {
    var arrInputs = files;
    for (var i = 0; i < arrInputs.length; i++) {
        var oInput = arrInputs[i];
        console.log(oInput.type)
            var sFileName = oInput.name;
            if (sFileName.length > 0) {
                var blnValid = false;
                for (var j = 0; j < _validFileExtensions.length; j++) {
                    var sCurExtension = _validFileExtensions[j];
                    if (sFileName.substr(sFileName.length - sCurExtension.length, sCurExtension.length).toLowerCase() == sCurExtension.toLowerCase()) {
                        blnValid = true;
                        break;
                    }
                }
                
                if (!blnValid) {
                    alert("Désolez, " + sFileName + " n'est pas valide, les extensions autorisées sont : " + _validFileExtensions.join(", "));
                    return false;
                }
            }

        return false;
    }
  
    return true;
}

var dropper = document.querySelector('#deplacemoi');

    dropper.addEventListener('dragover', function(e) {
        e.preventDefault(); // Annule l'interdiction de "drop"
    }, false);

    dropper.addEventListener('drop', function(e) {
        e.preventDefault();

        var files = e.dataTransfer.files,
            filesLen = files.length,
            mimeType = [],
            filenames = "";
        if(validate(files)){
            for(var i = 0 ; i < filesLen ; i++) {
                mimeType[i] = files[i].type;
                filenames += '\n' + files[i].name + ' type : ' + mimeType[i];
            }
            console.log(files.length + ' fichier(s) :\n' + filenames);
        }else{
            console.log("Extension non autorisée");
        }
    }, false);