Why are jpg files saved with .jpeg extension instead of .jpg?

Question

Browsers

Recently, on macOS, after updating Mozilla Firefox to versions above 50, JPG images are saved with the addition of the .jpeg extension.
Many applications do not accept this extension. This is annoying, because it used to be .jpg
Maybe someone came across? Tried the following:
1) Loading without add-ons
2) Deleting or editing mimeType
3) Creating a new profile

Answer 1

You have not tried to roll back Mozilla to a version below 50, maybe it's all about it?