magicmime 
magicmime is a Go package which allows you to discover a file's mimetype by looking for magic numbers in its content. It could be used as a supplementary for Go's mime package which only interprets the file extension to detect mimetypes. Internally, it implements libmagic(3) bindings.
Tested on Linux and Mac OS X, should be working on BSD. You could be able to build and make it working with Cygwin on Windows.
Prerequisites
This uses the libmagic library from the file tool, so you might need to install the development package for libmagic. On Debian or Ubuntu: apt-get install libmagic-dev. On RHEL, CentOS or Fedora: yum install file-devel. On Mac OS X: brew install libmagic. If you don't have the required package, compilation will be terminated by an error saying magic.h cannot be found.
Usage
In order to start, go get this repository:
go get github.com/rakyll/magicmime
Examples
See godoc examples.
Docs
https://godoc.org/github.com/rakyll/magicmime
License
Copyright 2013 Google Inc. All Rights Reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.