file.jpg or file.JPG or file.jpeg -- Does it matter?

JPG or jpg

• Yes. If your filename is "beach.JPG" then you need to reference it as "beach.JPG" -- having "beach.jpg" in the URL won't work!
• No. As long as the case of your filenames match the case in your URL references you'll be fine.

jpeg or jpg

• No. Media types can be mapped to multiple file extensions, so generally either picture.jpg or picture.jpeg filenames are fine.
  And for JPEG images, the media type (also known as MIME type) is "image/jpeg"