当源为多部分时，image.Decode导致“未知格式”。File

I have a multipart.File that a user uploads to my server, and then I take that file and upload it to s3 using aws-sdk-go, but I also want to create a thumbnail of that image.

The code below works fine in my tests when I file is the return value of an os.Open(... of a local file, but it hit's the err block when I send CreateThumbnail the same variable I sent to s3, which asks for an io.Reader

import (
  "image"
  "image/jpeg"
)
func UploadToS3(file multipart.File, /*snip*/) {
  _, uploadErr := uploader.Upload(&s3manager.UploadInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
    Body:   file,
    ContentType: aws.String(mimeType),
    ACL: aws.String("public-read"),
  })
  reader, err := CreateThumbnail(file)
}    

func CreateThumbnail(imageFile io.Reader) (io.Reader, error) {
  decodedImage, _, err := image.Decode(imageFile)
  if err != nil {
   fmt.Println("Error decoding", err, decodedImage) // "unknown format"
   return nil, err
 }
 /*snip*/

Most of the answers I see for the problem involve adding import _ "image/jpeg", but that's already imported (as well as png and gif). I'm fairly new to Golang, so I'm a bit lost as to what I'm doing wrong. I tried image.Decode(bufio.NewReader(imageFile)) as well, but that results in the same err.

The call uploader.Upload reads to the end of the file. Seek back to the beginning of the file before calling CreateThumbnail:

func UploadToS3(file multipart.File, /*snip*/) {
  _, uploadErr := uploader.Upload(&s3manager.UploadInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
    Body:   file,
    ContentType: aws.String(mimeType),
    ACL: aws.String("public-read"),
  })
  // Seek back to beginning of file for CreateThumbnail
  if _, err := file.Seek(0, 0); err != nil {
    // handle error
  }
  reader, err := CreateThumbnail(file)
}