Manual:$wgAllowTitlesInSVG/de-formal

Details
If true, the text will be allowed in files with a "svg" file extension or "image/svg+xml" MIME type.

MediaWiki will reject HTML-esque tags (including ) in uploaded files, as some browsers can't perform basic stuff like MIME detection and are therefore vulnerable to displaying valid image files as HTML and doing insecure things. You should leave this set to false if your web server is misconfigured and doesn't send appropriate MIME types for SVG images.

What are appropriate MIME types for SVG images? Well, that is currently under debate at the W3C, but you should see  in response headers when making a GET request for an svg file. You can easily check your server using the W3C's online SVG check tool. Or, from the command-line with curl:

HTTP/1.1 200 OK Date: Sun, 11 Feb 2018 05:01:23 GMT Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips PHP/5.6.33 X-Powered-By: PHP/5.6.33 X-Content-Type-Options: nosniff Last-Modified: Sat, 01 Jul 2017 15:50:20 GMT Content-Length: 69832 Content-Type: image/svg+xml