The list above isn't exhaustive, as there are many other modes and methods of saving replays. But it may give some idea and help to those wanting to post replays on ManiaExchange after the release of ManiaPlanet 4.

REPLAYS YOU CAN UPLOAD TO MX:

• Local Play -> Single map replays saved manually at the end of a run.
  MAPNAME_DRIVERNAME(00'00''00).Replay.Gbx
  
  
• Local Play -> Single map replays saved automatically.
  AutoSave_PersonalBest_MAPNAME.Replay.Gbx
  
  
• Local Play -> Single map replays saved automatically.
  DRIVERNAME_TITLENAME@TITLEAUTHOR_MAPNAME.Replay.Gbx
  
  
• Online replays saved automatically on a Time Attack server with "Save automatically all replays" turned on in the title profile settings.
  AutoSave_Replay_DATE-TIME_MAPNAME_DRIVERNAME(00'00''00).Replay.Gbx
  
  
• Online replays saved automatically on a Rounds server with "Save automatically all replays" turned on in the title profile settings.
  AutoSave_Replay_DATE-TIME_MAPNAME_DRIVERNAME(00'00''00).Replay.Gbx
  
  
• Online replay saved automatically on a Rounds server.
  AutoSave_PersonalBest_MAPNAME.Replay.Gbx

REPLAYS YOU CAN NOT CURRENTLY UPLOAD TO MX:

• Online replays saved using "ESC -> Advanced Options -> Save Previous Replay" while on Time Attack server.
  MAPNAME_DRIVERNAME(00'00''00).Replay.Gbx
  
  
• Online replays saved automatically on a Time Attack server.
  AutoSave_PersonalBest_MAPNAME.Replay.Gbx
  
  
• Autosave replays from your validation run after building a track.
  AutoSave_Replay_DATE-TIME_MAPNAME_DRIVERNAME(00'00''00).Replay.Gbx
  
  
• Replays driven in a different titlepack than the map was built in.
  Example: TMStadium map. Replay driven in ESL titlepack.

You can find your replays in:
Documents\ManiaPlanet\Replays\

Autosave replays are in:
Documents\ManiaPlanet\Replays\Autosaves\

It helps to sort by last modified when looking for a recently driven replay.

Info taken from my post on MP Forums here:
https://forum.maniaplanet.com/viewtopic.php?p=284856#p284856

Post last updated: May 24, 2017

Doing some continued testing with bcs'Flame, because it appears some of the scenarios I said that worked actually produce invalid replays. We'll post more info soon.