code: @Nonnull
    public FineUploader5Validation addAllowedExtensions (@Nullable final Collection <String> aAllowedExtensions)
    {
      if (aAllowedExtensions != null)
        m_aValidationAllowedExtensions.addAll (aAllowedExtensions);
      return this;
    }
docs: Specify file valid file extensions here to restrict uploads to specific
  types.
  
  @param aAllowedExtensions
  The allowed extensions to be added.
  @return this for chaining