Only use getimagesize() for GIFs, JPGs and PNGs.

[leofeyer]

So SVGs and other images are not unnecessarily run through getimagesize().

[ausi]

getimagesize() supports much more image formats than just the three major ones: jpeg2000, swf, psd, bmp, wbmp, xbm, tiff_ii, tiff_mm, iff, jb2, jpc, jp2, jpx, swc, ico, webp.

I think the case for a different format than those (apart from SVG) is very uncommon and calling getimagesize() on a different format produces not much overhead IMO.

The case for SVG images should get solved by #23.

[leofeyer]

getimagesize() supports much more image formats than just the three major ones: jpeg2000, swf, psd, bmp, wbmp, xbm, tiff_ii, tiff_mm, iff, jb2, jpc, jp2, jpx, swc, ico, webp.

Yes, but that strongly depends on the GDlib version. E.g. webp support has only been added in one of the most recent versions, therefore the only formats that can be handled reliably by all versions are gif, jpg and png (and bmp).

[leofeyer]

But anyway, #23 looks promising, therefore I'm closing this PR.