Manual:$wgAllowTitlesInSVG

Details
If true, will be allowed in files with an "image/svg" MIME type.

MediaWiki will reject HTMLesque tags in uploaded files as some browsers can't perform basic stuff like MIME detection and are therefore vulnerable to uploading dangerous/invalid files as images. You should leave this disabled if your web server is mis-configured and doesn't send appropriate MIME types for SVG images.