Code:play 

package main

import (
	"bytes"
	"fmt"
	"os"

	"github.com/gabriel-vasile/mimetype"
)

func main() {
	testBytes := []byte("This random text should have a MIME type of text/plain; charset=utf-8.")

	mime := mimetype.Detect(testBytes)
	fmt.Println(mime.Is("text/plain"), mime.String(), mime.Extension())

	mime, err := mimetype.DetectReader(bytes.NewReader(testBytes))
	fmt.Println(mime.Is("text/plain"), mime.String(), mime.Extension(), err)

	mime, err = mimetype.DetectFile("a nonexistent file")
	fmt.Println(mime.Is("application/octet-stream"), mime.String(), os.IsNotExist(err))
}

Output:

true text/plain; charset=utf-8 .txt
true text/plain; charset=utf-8 .txt <nil>
true application/octet-stream true

When detecting from an io.Reader, mimetype will read the header of the input. This means the reader cannot just be reused (to save the file, for example) because the header is now missing from the reader.

If the input is a pure io.Reader, use io.TeeReader, io.MultiReader and bytes.Buffer to create a new reader containing the whole unaltered data.

If the input is an io.ReadSeeker, call reader.Seek(0, io.SeekStart) to rewind it.

Code:play 

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"

	"github.com/gabriel-vasile/mimetype"
)

func main() {
	// Set header size to 10 bytes for this example.
	mimetype.SetLimit(10)

	testBytes := []byte("This random text should have a MIME type of text/plain; charset=utf-8.")
	inputReader := bytes.NewReader(testBytes)

	// buf will store the 10 bytes mimetype used for detection.
	header := bytes.NewBuffer(nil)

	// After DetectReader, the first 10 bytes are stored in buf.
	mime, err := mimetype.DetectReader(io.TeeReader(inputReader, header))

	// Concatenate back the first 10 bytes.
	// reusableReader now contains the complete, original data.
	reusableReader := io.MultiReader(header, inputReader)
	text, _ := ioutil.ReadAll(reusableReader)
	fmt.Println(mime, bytes.Equal(testBytes, text), err)
}

Output:

text/plain; charset=utf-8 true <nil>

Considering the definition of a binary file as "a computer file that is not a text file", they can differentiated by searching for the text/plain MIME in their MIME hierarchy.

Code:play 

package main

import (
	"fmt"

	"github.com/gabriel-vasile/mimetype"
)

func main() {
	testBytes := []byte("This random text should have a MIME type of text/plain; charset=utf-8.")
	detectedMIME := mimetype.Detect(testBytes)

	isBinary := true
	for mime := detectedMIME; mime != nil; mime = mime.Parent() {
		if mime.Is("text/plain") {
			isBinary = false
		}
	}

	fmt.Println(isBinary, detectedMIME)
}

Output:

false text/plain; charset=utf-8

Code:play 

package main

import (
	"fmt"

	"github.com/gabriel-vasile/mimetype"
)

func main() {
	testBytes := []byte("This random text should have a MIME type of text/plain; charset=utf-8.")
	allowed := []string{"text/plain", "application/zip", "application/pdf"}
	mime := mimetype.Detect(testBytes)

	if mimetype.EqualsAny(mime.String(), allowed...) {
		fmt.Printf("%s is allowed\n", mime)
	} else {
		fmt.Printf("%s is now allowed\n", mime)
	}
}

Output:

text/plain; charset=utf-8 is allowed