จะให้อัพโหลดไฟล์เป็นนามสกุล PDF อย่างเดียว ทำยังงัยคะ

$file_type = $_FILES['fileUpload']['type']; //returns the mimetype

$allowed = array("image/jpeg", "image/gif", "application/pdf");
if(!in_array($file_type, $allowed)) {
  $error_message = 'Only jpg, gif, and pdf files are allowed.';
}

<input type="file" id="file" onchange="return fileValidation()"/>

function fileValidation(){
    var fileInput = document.getElementById('file');
    var filePath = fileInput.value;
    var allowedExtensions = /(\.jpg|\.jpeg|\.png|\.gif)$/i;
    if(!allowedExtensions.exec(filePath)){
        alert('Please upload file having extensions .jpeg/.jpg/.png/.gif only.');
        fileInput.value = '';
        return false;
    }else{
        //Image preview
        if (fileInput.files && fileInput.files[0]) {
            var reader = new FileReader();
            reader.onload = function(e) {
                document.getElementById('imagePreview').innerHTML = '<img src="'+e.target.result+'"/>';
            };
            reader.readAsDataURL(fileInput.files[0]);
        }
    }
}