Json upload mode (#21) * Refactor parameter collection to ignore upload mode and add upload mode retrieval function * Remove upload mode handling from parameter collection and related functions * Add base64 download handler and refactor existing download functions * Add error handling test for PlainDownloadHandler and implement StoreRawForTesting method * Add tests for PlainDownloadHandler with base64 decoding scenarios * Enhance base64 decoding in download handler and update HTML documentation for advanced mode * Update README to include base64 encoding option for data retrieval * Update README to clarify key pair usage and enhance feature descriptions