Rendering png with transparent backgrounds

Support Lighting and Rendering
1

In the past, I have been able to render png files with transparent backgrounds by checking “transparent” in the Film panel. Now it is no longer working. The backgrounds look transparent in the render preview, but the png file has black where it ought to be transparent. I can’t figure out what the problem is.

Thanks for any clues.

2

“RGBA” is highlighted in the file output?
Other than that: Please ALWAYS supply an example .blend file with any support question, as this is not supposed to be a guessing game.

3

Thank you for your help. I assumed there was an easy answer which didn’t require a file to be submitted. I was right.