Override MIME type to match MediaStore behavior.

Internally, DownloadManager synchronizes its contents with MediaStore,
which relies heavily on using file extensions to determine MIME types.

To prevent these two databases from getting confused, this change
adjusts DownloadManager to force the MIME type of already-completed
downloads based on the file extension, matching what MediaStore does.

Bug: 159594536
Test: atest PublicApiAccessTest#testAddCompletedWithoutExtension
Change-Id: I60c613eafcfe55007dffcac2d7d1fe375b753c19