Breaking this of related T34660, which was broken off of T6421. This would also solve T31284.
As MW detects the MIME type of the file as it is being uploaded, it should not rely on the uploader to provide a file extension. Rather the file type should be set automatically by the software. Any extension detected in the name should be automatically removed.
For example if `Cheese.JPEG` is uploaded, but the MIME type is `PNG`, the file should be named `Cheese.png`, and not `Cheese.JPEG.png`. If that MIME type is correct, it should simply be named `Cheese.jpg`. This should also create a notice for the uploader, so they don't lose track of their uploaded file.
Obviously this will not fix existing issues mentioned in the first two bugs, but it will prevent future issues.