The contents of the multipart.File are as follows: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD..."

I'm trying to get it into the Image.DecodeConfig() method like this:

```
import (
    "image"
    "image/jpeg"
    "io"
)

func ImgCheckSize(file io.Reader) (io.Reader, error) {
    config, format, err := image.DecodeConfig(file)
...

```

The error prints: "image: unknown format"

I've also printed out the multipart.FileHeader.Header which reads:

```
map[Content-Disposition:[form-data; name="img1"; filename="img1"] Content-Type:[application/octet-stream]]

```

Has anyone come across this before? Any useful suggestions would help! Many many thanks