To get the file size and MIME type:

// Size constants
const (
        MB = 1 << 20
)

type Sizer interface {
        Size() int64
}

func Sample(w http.ResponseWriter, r *http.Request) error {
        if err := r.ParseMultipartForm(5 * MB); err != nil {
                return err
        }

        // Limit upload size
        r.Body = http.MaxBytesReader(w, r.Body, 5*MB) // 5 Mb

        //
        file, multipartFileHeader, err := r.FormFile("file")

        // Create a buffer to store the header of the file in
        fileHeader := make([]byte, 512)

        // Copy the headers into the FileHeader buffer
        if _, err := file.Read(fileHeader); err != nil {
                return err
        }

        // set position back to start.
        if _, err := file.Seek(0, 0); err != nil {
                return err
        }

        log.Printf("Name: %#v\n", multipartFileHeader.Filename)
        log.Printf("Size: %#v\n", file.(Sizer).Size())
        log.Printf("MIME: %#v\n", http.DetectContentType(fileHeader))
}

Sample output:

2016/12/01 15:00:06 Name: "logo_35x30_black.png"
2016/12/01 15:00:06 Size: 18674
2016/12/01 15:00:06 MIME: "image/png"