This was detected while testing MDL-71240

To reproduce:

• Setup the TeX filter to produce .svg graphics (and enable it).
• Disable the MathML filter.
• Ensure that you don't have the latex binary defined.
• Then the mimetex fallback will be used.
• Add any formula anywhere (a page, a forum post...).
• You see the formula and, looking to html source it's meant to be a .svg (also http headers...)
• But really, if you download it... it's a gif (because mimetex only generates gifs).