if cksum == nil {
			continue
		}
		if cksum.Valid() {
			w.Header().Set(cksum.Type.Key(), v)
		}
	}
}

// GetContentChecksum returns content checksum.
// Returns ErrInvalidChecksum if so.
// Returns nil, nil if no checksum.
func GetContentChecksum(h http.Header) (*Checksum, error) {
	if trailing := h.Values(xhttp.AmzTrailer); len(trailing) > 0 {
		var res *Checksum
		for _, header := range trailing {

// Returns ErrInvalidChecksum if a problem with the checksum is found.
func (r *Reader) AddChecksum(req *http.Request, ignoreValue bool) error {
	cs, err := GetContentChecksum(req.Header)
	if err != nil {
		return ErrInvalidChecksum
	}
	if cs == nil {
		return nil
	}
	r.contentHash = *cs
	if cs.Type.Trailing() {
		r.trailer = req.Trailer
	}

		apiErr.Description = fmt.Sprintf("%s (%v)", apiErr.Description, errors.New("The file or text content is missing"))
		writeErrorResponse(ctx, w, apiErr, r.URL)
		return
	}
	checksum, err := hash.GetContentChecksum(formValues)
	if err != nil {
		apiErr := errorCodes.ToAPIErr(ErrMalformedPOSTRequest)
		apiErr.Description = fmt.Sprintf("%s (%v)", apiErr.Description, fmt.Errorf("Invalid checksum: %w", err))