Mixed tile format doesn't work for S3 and FS #169

Describe the bug
When working with files, the merger uses file extension with no regard to existing file extension. This means that if a PNG file is merged with a new JPEG resulting in a full tile (JPEG) then it will be stored and both files would exist (for the same x,y,z).

To Reproduce

1. Hold a tile with transparency in FS or S3
2. Update with a tile that should result in a tile with no transparency
3. New tile is written next to the existing tile

Expected behavior
Only the new calculated tile should exist.

Additional context
To combat this mapproxy uses .mixed extension, this way at the end the correct data would be placed. This is because the file extension isn't needed for discovering the tile format (can be understood from tile data).
If we implement the same strategy then we will be able to use mixed format with Mapproxy serving as well.