When the user goes to save an image without a file extension in Firefox, for example one that is base64 encoded, she might see something like this:

This is annoying; most software won't recognise it as a valid jpeg file, and it's a lot on uncessesary manual work to change it each time she downloads an image.

She can fix this by editing the file /etc/mime.types; Firefox will take the first value listed against the image/jpeg entry, so we just need to swap the order around:

image/jpeg   jpg jpeg jpe

And now when she saves a file it works as expected!