When checking mime type of SVG files, be forgiving if they lack an XML declaration

[jcherniak]

When trying to read SVG files to pass through the svg() TWIG function, if they lack a XML declaration, they are not read as FileHelper::isSvg returns false.

Given the comment in FileHelper::getMimeType, this seemed like the appropriate place to make the change. Otherwise, you could allow 'image/svg' in the FileHelper::isSvg.

Either way, this allows you to reference local SVG files that lack an XML declaration.

To reproduce:

1. Create a local asset volume.
2. Create an Asset field linking to it.
3. Upload this SVG into the folder and pick it in the asset picker: alarm-clock.svg.txt (Rename it to alarm-clock.svg, renamed due to Github not allowing SVG uploads).
4. In your template, add the following twig code:
   {{ svg(icon.folder.volume.path ~ "/" ~ icon.filename) }}
5. Notice the function returns an empty string rather than the content of the file.

Note: this is an icon from Font-Awesome Pro, so adding XML declarations to all of the 800+ files is not practical. Given the comment in FileHelper::getMimeType, it seems that was supposed to cover SVG files anyway.

[brandonkelly]

Strange - I believe the only valid SVG MIME type is image/svg+xml, so something may be messed up with your system’s magic file (the file that provides patterns that help PHP identify what a given file’s MIME type is, based on its contents). In my case, alarm-clock.svg did correctly come back as image/svg+xml.

That said, I’ve fixed this for the next release by updating FileHelper::isSvg() to just make sure that the MIME type starts with image/svg, rather than specifically checking for image/svg+xml.