code: func (t Types) GetByMainSubType(mainType, subType string) (tp Type, found bool) {
  	for _, tt := range t {
  		if strings.EqualFold(mainType, tt.MainType) && strings.EqualFold(subType, tt.SubType) {
  			if found {
  				// ambiguous
  				found = false
  				return
  			}
  
  			tp = tt
  			found = true
  		}
  	}
  	return
  }
docs: // GetByMainSubType gets a media type given a main and a sub type e.g. "text" and "plain".
  // It will return false if no format could be found, or if the combination given
  // is ambiguous.
  // The lookup is case insensitive.