code: @Nonnull
    public FineUploader5Validation setAllowedExtensions (@Nullable final Collection <String> aAllowedExtensions)
    {
      m_aValidationAllowedExtensions.setAll (aAllowedExtensions);
      return this;
    }
docs: Specify file valid file extensions here to restrict uploads to specific
  types.
  
  @param aAllowedExtensions
  The allowed extensions to be set.
  @return this for chaining