Manual:$wgAllowTitlesInSVG/en

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