function allow_upload_custom_files($mimes) {
    $mimes['html'] = 'text/html';
    $mimes['json'] = 'application/json';
    return $mimes;
}
add_filter('upload_mimes', 'allow_upload_custom_files');
<title>XML Sitemap</title><style type="text/css">
					body {
						font-family:"Lucida Grande","Lucida Sans Unicode",Tahoma,Verdana;
						font-size:13px;
					}
					
					#intro {
						background-color:#cfebf7;
						border:1px #2580B2 solid;
						padding:5px 13px 5px 13px;
						margin:10px;
					}
					
					#intro p {
						line-height:16.8667px;
					}
					#intro strong {
						font-weight:normal;
					}
					
					td {
						font-size:11px;
					}
					
					th {
						text-align:left;
						padding-right:30px;
						font-size:11px;
					}
					
					tr.high {
						background-color:whitesmoke;
					}
					
					#footer {
						padding:2px;
						margin-top:10px;
						font-size:8pt;
						color:gray;
					}
					
					#footer a {
						color:gray;
					}
					
					a {
						color:black;
					}
				</style><h1>XML Sitemap Index</h1><div id="intro"><p>
                This XML sitemap is used by search engines which follow the <a rel="external nofollow" href="https://sitemaps.org">XML sitemap standard</a>. This file contains links to sub-sitemaps, follow them to see the actual sitemap content.
            </p><p>
                This file was dynamically generated using the <a rel="external nofollow" href="https://wordpress.org/">WordPress</a> content management system and <strong><a rel="external nofollow" href="https://auctollo.com/products/google-xml-sitemap-generator/" title="XML Sitemap Generator for Google">XML Sitemap Generator for Google</a></strong> by <a rel="external nofollow" href="https://auctollo.com/">Auctollo</a>.
            </p></div><div id="content"><table><tr style="border-bottom:1px black solid"><th>URL of sub-sitemap</th><th>Last modified (GMT)</th></tr><tr><td><a href="https://muramural3.com/sitemap-misc.html">https://muramural3.com/sitemap-misc.html</a></td><td>2025-07-24T17:03:14+00:00</td></tr><tr class="high"><td><a href="https://muramural3.com/post-sitemap.html">https://muramural3.com/post-sitemap.html</a></td><td>2025-07-24T17:03:14+00:00</td></tr><tr><td><a href="https://muramural3.com/page-sitemap.html">https://muramural3.com/page-sitemap.html</a></td><td>2025-06-30T03:51:17+00:00</td></tr></table></div><div id="footer"><p>
						Dynamically generated with <a rel="external nofollow" href="https://auctollo.com/products/google-xml-sitemap-generator/" title="XML Sitemap Generator for Google">XML Sitemap Generator for Google</a> by <a rel="external nofollow" href="https://auctollo.com/">Auctollo</a>. This XSLT template is released under the GPL and free to use.
            		</p><p>
	                    If you have problems with your sitemap please visit the <a rel="external nofollow" href="https://auctollo.com/products/google-xml-sitemap-generator/help/" title="Frequently Asked Questions">FAQ</a> or the <a rel="external nofollow" href="https://wordpress.org/support/plugin/google-sitemap-generator">support forum</a>.
            		</p></div>