When saving an image to the disk a file dialogue emerges, where the extensions are written in capital letters, e.g. .JPG instead of .jpg etc… Please have this corrected.
It depends on how it is specified in the message code. If it is uppercase, that is how it will be saved.
Content-Disposition: attachment; filename="Screenshot2.JPG"
It will be exactly how the filename is specified there.
You are right. It depends on the message, but I do not understand why this occurs in the special case where my email containt only an image?
Now I found out. The sender gave a name to the image with the extension in capital letters. It must be a bug of mail on iOS.