Patch for skins/common/upload.js
IMHO upload form should make file extensions lowercase automatically during file name editbox autofill.
Patch is attached.
T42479: File extensions should be automatically decided by MIME type at upload
T34660: File extensions for the same file type should not allow variations of a file name (File:X.jpg, File:X.jpeg, File:X.JPG should all refer to the same file)
T31284: Upload form should change file extensions to the canonical form automatically (lowercase, jpeg→jpg etc.)
T213484: Normalize file extensions (capital vs small letters; jpg vs jpeg) for new uploads on Commons