By default, the Attachment is hidden in list form. We can add the CSS style into script editor web part in display/new form page.
<style type="text/css">
#idAttachmentsRow{
display: black !important;
}
</style>

If you don't want to use the Ribbon to attach file, we can use the code below to add the attach file control into a row of table.
<script src="https://code.jquery.com/jquery-1.12.4.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(function(){
var attachHTML='<tr><td width="190" height="15" class="ms-formlabel" valign="top">Attach File</td><td height="15" class="ms-formbody" id="attachmentsOnClient" valign="bottom"><span dir="ltr"><input name="fileupload0" title="Name" class="ms-fileinput" id="onetidIOFile" type="file" size="56"></span></td></tr><tr><td> </td><td><input class="ms-ButtonHeightWidth" id="attachOKbutton" onclick="OkAttach()" type="button" value="Attach"><span class="ms-SpaceBetButtons" id="idSpace"></span><input class="ms-ButtonHeightWidth" id="attachCancelButton" accesskey="C" onclick="CancelAttach()" type="button" value="Cancel"></td></tr>';
$(".ms-formtable").append(attachHTML);
})
</script>
