I added extra logic to my code.
Get ContentType from Extension first.

fileType := mime.TypeByExtension(path.Ext(filePath))
if fileType == "" {
    fileType = http.DetectContentType(buffer)
}

It fix my problem, although don't feel nice.

If you feel don't need the feature. Please close this.
Sorry for the noise.

Marking as 1.8 to decide whether or not to do it in 1.8. It would help to know whether other systems, such as the file command, recognize SVG files, and how they do it.

@ianlancetaylor

Landed here with the same problem. The file command correctly produces image/svg+xml as an SVG file's mime type:

$ file -i /tmp/foo.svg
/tmp/foo.svg: image/svg+xml; charset=us-ascii

http.DetectContentType is documented to be an implementation of https://mimesniff.spec.whatwg.org/ and that mimesniff spec doesn't define svg.

And as you found, mime.TypeByExtension has ".svg" in its built-in table:

https://github.com/golang/go/blob/release-branch.go1.6/src/mime/type.go#L35

So I don't think there's anything to do here.

@bradfitz actually I reported an issue to whatwg whatwg/mimesniff#7, when this issue was filed here so that perhaps if they could define its spec, they could work on it, add it to the spec and we could add it in here. The issue that I filed is still open, but from the looks of it doesn't seem like they'll be adding it in. If they add it in, I'll ping this bug and we can see what we can.