Add stamp, fix html, change accepts

src/main/resources/templates/convert/markdown-to-pdf.html

@@ -13,7 +13,7 @@
                     <div class="col-md-6">
                         <h2 th:text="#{MarkdownToPDF.header}"></h2>
                         <form method="post" enctype="multipart/form-data" th:action="@{api/v1/convert/markdown/pdf}">
-                            <div th:replace="~{fragments/common :: fileSelector(name='fileInput', multiple=false)}"></div>
+                            <div th:replace="~{fragments/common :: fileSelector(name='fileInput', multiple=false, accept='text/markdown')}"></div>
                             <br>
                             <button type="submit" id="submitBtn" class="btn btn-primary" th:text="#{MarkdownToPDF.submit}"></button>