Currently progressive rendering previews are hard-coded as JPEG. This is fine for most render jobs, but sometimes transparency is used and thus rendering to PNG is required. The easiest way to give artists control over this would be to use the output settings of the blend file.
- Update the Cloud add-on to emit a warning/error when an output format is selected that's incompatible with FFMPEG and thus unsuitable for preview generation (which includes EXR because they are useless as preview-versions of EXR files).
- Create a new preview_exr_sequence command that basically does the same as exr_sequence_to_jpeg except it doesn't mention the output file format in the name and uses the settings in the blend file.
- Update the preview-handing tasks to use the configured output extension (rather than hardcoded .jpg)