Ok, I have tried several variables at once in the email body:
A new PO file has been uploaded.
{TRIGGER(Active|LastTrigger|RemoteFile.Result.Name)}
{TASK(Active,Result.DownloadedFilesNames)}
{TASK(Active,Result.UploadedFilesNames)}
{TASK(ffac0f64-8650-42c1-ae97-fe79b86e61ce,Result.UploadedFilesNames)}
{TASK(ffac0f64-8650-42c1-ae97-fe79b86e61ce,Result.DownloadedFilesNames)}
The email displayed like this:
A new PO file has been uploaded.
Error in argument
(No match on Variable: TASK(Active,Result.DownloadedFilesNames) for this task type)
(No match on Variable: TASK(Active,Result.UploadedFilesNames) for this task type)
520140828124239_XXX51203_PRMS_.xlsx
520140828124239_XXX51203_PRMS_.xlsx
520140828124239_XXX51203_PRMS_.xlsx
It seems the last three variables worked, but I am not sure what really happened.