Getting an Images Type and Size in Golang (or any other language)

Go's image libraries aren't the most mature. They fail to work for a number of common encodings/formats/whatever. This means that if you want to get basic information about a file, you'll need to rely on something else. Well, incase all you need to know is an image's format or its dimensions, consider using this [probably very buggy] code:

func getFormat(file *os.File) (string) {
  bytes := make([]byte, 4)
  n, _ := file.ReadAt(bytes, 0)
  if n < 4 { return "" }
  if bytes[0] == 0x89 && bytes[1] == 0x50 && bytes[2] == 0x4E && bytes[3] == 0x47 { return "png" }
  if bytes[0] == 0xFF && bytes[1] == 0xD8 { return "jpg" }
  if bytes[0] == 0x47 && bytes[1] == 0x49 && bytes[2] == 0x46 && bytes[3] == 0x38 { return "gif" }
  if bytes[0] == 0x42 && bytes[1] == 0x4D { return "bmp" }
  return ""
}

func getGifDimensions(file *os.File) (width int, height int) {
  bytes := make([]byte, 4)
  file.ReadAt(bytes, 6)
  width = int(bytes[0]) + int(bytes[1]) * 256
  height = int(bytes[2]) + int(bytes[3]) * 256
  return
}

func getBmpDimensions(file *os.File) (width int, height int) {
  bytes := make([]byte, 8)
  file.ReadAt(bytes, 18)
  width = int(bytes[3]) << 24 | int(bytes[2]) << 16 | int(bytes[1]) << 8 | int(bytes[0])
  height = int(bytes[7]) << 24 | int(bytes[6]) << 16 | int(bytes[5]) << 8 | int(bytes[4])
  return
}

func getPngDimensions(file *os.File) (width int, height int) {
  bytes := make([]byte, 8)
  file.ReadAt(bytes, 16)
  width = int(bytes[0]) << 24 | int(bytes[1]) << 16 | int(bytes[2]) << 8 | int(bytes[3])
  height = int(bytes[4]) << 24 | int(bytes[5]) << 16 | int(bytes[6]) << 8 | int(bytes[7])
  return
}

func getJpgDimensions(file *os.File) (width int, height int) {
  fi, _ := file.Stat()
  fileSize := fi.Size()

  position := int64(4)
  bytes := make([]byte, 4)
  file.ReadAt(bytes[:2], position)
  length := int(bytes[0] << 8) + int(bytes[1])
  for position < fileSize {
    position += int64(length)
    file.ReadAt(bytes, position)
    length = int(bytes[2]) << 8 + int(bytes[3])
    if (bytes[1] == 0xC0 || bytes[1] == 0xC2) && bytes[0] == 0xFF && length > 7 {
      file.ReadAt(bytes, position+5)
      width = int(bytes[2]) << 8 + int(bytes[3])
      height = int(bytes[0]) << 8 + int(bytes[1])
      return
    }
    position += 2
  }
  return 0, 0
}