I think this is something which happens within Froala and is likely related to how the raw paste data has to be manipulated into a format that can be uploaded and displayed in the editor. There's a lot of variables here and a lot of different approaches built in to the editor to handle all sorts of edge cases. In some cases, it might be possible for it to detect and retain the original extension, but the assumption is if it doesn't do that then there must be a specific reason for it.
There is no actual conversion of the data in this case. If you upload a jpg file, it will still be a jpg file, but it will just give it a png extension. This is actually sort of similar to how we process avatars - any avatar you upload will be given a .jpg extension.
Ultimately, however, the extension makes very little difference to anything. Browsers, image editors, etc. they don't rely on the extension to decide how to display an image. They know from the internals of the image, the headers and any metadata within the image what the actual image type is.
So to answer your original question (before you edited the first post), no, it isn't better to save it as a jpg. The file you upload will be identical to the one that is saved, regardless of the extension.