howood/imagereductor

package utils

import (
    "bytes"
    "io"
    "net/http"
)

// GetContentTypeByReadSeeker is get content type by Readseeker
func GetContentTypeByReadSeeker(reader io.ReadSeeker) (string, error) {
    _, err := reader.Seek(0, io.SeekStart)
    if err != nil {
        return "", err
    }
    buf := new(bytes.Buffer)
    _, err = buf.ReadFrom(reader)
    if err != nil {
        return "", err
    }
    return http.DetectContentType(buf.Bytes()), nil
}