I don't think the image is converted to a PNG - I think it already is one.
The rich-text editor automatically creates a name for the uploaded file when one isn't available and it does this based on the MIME type of the image.
If I rename a file on my computer from image.jpg
to image.png
and paste it to upload it, the file name comes out as <current_time_stamp>.jpg
because the MIME type of the image is image/jpg
.
The extension of a file has very little bearing on how an image is handled meaning that most browsers and software totally ignore what the extension is.
So, ultimately, even if this was unexpected, it wouldn't make any difference.