JPEG plugin should warn that it can't export transparent pixels

Operating System: all

Description of the feature

Once or twice a week i see people ask - e.g on social media - why GIMP didn't save the transparency in their image. Almost always they are exporting to JPEG, and exporting instead to PNG solves their problem.

Maybe a drop down in the JPEG plug-in would help, such as,

Transparency: [ convert to white | convert to background colour | convert to black | draw a chess board ] with a tooltip to say, The JPEG format does not support transparency.

would be useful and would help? At the very least, if has_alpha is true in the save-dialog() could we add a message about not being able to save transparency?

It's true that e.g. the PNG plug-in doesn't warn about not saving layers but we don't get asked about that nearly as often