PutPart returning wrong size

[frodopwns]

I can successfully use PutAll with a MultiPart upload and I can do a Single part upload with no trouble.

When I create a []bye of the size I want my parts to be (say 5000000) and I try to upload a 25mb file in 5mb parts each part thinks it is 25mb instead of 5mb.

When uploading a 15m file I expect to see

Uploading...
Processing 1 part of 3 and uploaded 5242880.0 bytes.
 Processing 2 part of 3 and uploaded 5242880.0 bytes.
 Processing 3 part of 3 and uploaded 5242880.0 bytes.

Instead I see:

Uploading...
Processing 1 part of 3 and uploaded 15728640 bytes.
 Processing 2 part of 3 and uploaded 15728640 bytes.
 Processing 3 part of 3 and uploaded 15728640 bytes.
Is this due to an issue with the file.Read(partBuffer)? Any help would be much appreciated.

I am using go 1.5.1 on a mac.

package main

import (
    "bufio"
    "fmt"
    "math"
    "net/http"
    "os"

    "github.com/mitchellh/goamz/aws"
    "github.com/mitchellh/goamz/s3"
)

func check(err error) {
    if err != nil {
        panic(err)
    }
}

func main() {
    fmt.Println("Test")

    auth, err := aws.GetAuth("XXXXX", "XXXXXXXXXX")
    check(err)

    client := s3.New(auth, aws.USWest2)

    b := s3.Bucket{
        S3:   client,
        Name: "some-bucket",
    }

    fileToBeUploaded := "testfile"
    file, err := os.Open(fileToBeUploaded)
    check(err)
    defer file.Close()

    fileInfo, _ := file.Stat()
    fileSize := fileInfo.Size()
    bytes := make([]byte, fileSize)

    // read into buffer
    buffer := bufio.NewReader(file)
    _, err = buffer.Read(bytes)
    check(err)
    filetype := http.DetectContentType(bytes)

    // set up for multipart upload
    multi, err := b.InitMulti("/"+fileToBeUploaded, filetype, s3.ACL("bucket-owner-read"))
    check(err)

    const fileChunk = 5242880 // 5MB
    totalPartsNum := uint64(math.Ceil(float64(fileSize) / float64(fileChunk)))
    parts := []s3.Part{}

    fmt.Println("Uploading...")
    for i := uint64(1); i < totalPartsNum; i++ {

        partSize := int(math.Min(fileChunk, float64(fileSize-int64(i*fileChunk))))
        partBuffer := make([]byte, partSize)

        _, err := file.Read(partBuffer)
        check(err)

        part, err := multi.PutPart(int(i), file) // write to S3 bucket part by part
        check(err)

        fmt.Printf("Processing %d part of %d and uploaded %d bytes.\n ", int(i), int(totalPartsNum), int(part.Size))
        parts = append(parts, part)
    }

    err = multi.Complete(parts)
    check(err)

    fmt.Println("\n\nPutPart upload completed")

}

Am I doing something wrong or is this a bug of some sort?