We have have a customer experiencing an the following issue:

Unable to find manifest. Error: No file with extension '.csv' found in container 'x' for path 'xyz'

The error is being reported here, so I started doing some digging. Turns out we do not change the file extension from our default. If we trace the default compression mode starting at line 275 . The customer does not have a json manifest so we just pass the default into the get_latest_cost_export_for_path. Where we check to make sure that the default is a valid option and then pass our default extension to _get_latest_blob_for_path where we call 
_get_latest_blob and discover we have no files ending a .csv because they have compression turned on for their export.