Consider using http.DetectContentType to inter the image type

[h2non]

Probably this algorithm will be more reliable than checking the first chunk of bytes of the image:
http://golang.org/src/net/http/sniff.go?s=648:690#L11

[lovell]

"more reliable than checking the first chunk of bytes"

If you're happy with the minimum version of libvips being 7.40.0+, I recommend using vips_foreign_find_load_buffer for this purpose as it performs some basic verification of image metadata also - see https://github.com/lovell/sharp/blob/judgement/src/common.cc#L40

[h2non]

That's a good idea and probably better than Go net/http implementation just because it only checks image types. I'll support it. The restriction with libvips minimum required version is not a big deal

[lovell]

Great, thanks Tomás.

vips_foreign_find_load_buffer has the added advantage of providing a run-time check of formats available at compile time. For example, if libvips is compiled without WebP support and is therefore unable to decode that format then this method won't detect it.

[h2non]

That's absolutely fantastic. In fact, I experimented some troubles yesterday with that because my original libvips compilation has no support for webp, so I was getting a nil pointer dereference runtime panic, which is very ugly. Using that function I can handle that cases better with a graceful error

[h2non]

@lovell I've finally supported your proposal:
https://github.com/h2non/bimg/blob/master/vips.go#L172

Thanks!