Two downloads of the same name collide incorrectly (BB-10684)

Uploading two files with the same name to the Downloads page results in a collision: Bitbucket will list both files, but when one file is deleted, trying to download the other will result in an HTTP 404 error, despite the link still existing on the Downloads page.

The behavior should be made consistent: either uploads of the same name should overwrite both the file and the link on the Downloads page, or multiple uploads of the same name should result in distinct Download links on the Downloads page and distinct files.