code: @Nonnull
    public FineUploaderBasic setAllowedExtensions (@Nullable final Set <String> aAllowedExtensions)
    {
      m_aValidationAllowedExtensions.setAll (aAllowedExtensions);
      return this;
    }
docs: This may be helpful if you want to restrict uploaded files to specific file
  types. Note that this validation option is only enforced by examining the
  extension of uploaded file names. For a more complete verification of the
  file type, you should use, for example, magic byte file identification on the
  server side and return {"success": false} in the response if the file type is
  not on your whitelist.
  
  @param aAllowedExtensions
  The allowed extensions to be set.
  @return this