No extension on filename

[Tiksi]

Describe the bug
When trying to download a file, the content-disposition header is being set the filename lacking the extension. If trying to download arbitrary file types (zip, rar, tar), this can be confusing if you don't know the file type ahead of time.

To Reproduce
Steps to reproduce the behavior:

1. Go to http://cdn.paste.click/fBbjLt7FifSV6cWTXoIDdQ or any other url in a browser
2. Try to "save as"
3. Notice the file doesn't have an extension

Expected behavior
The file should have the correct extension.

Additional context
GCS allows a Content-Disposition header to be set on objects, we should set this with the correct filename with the extension derived from the mimetype determined by mimemagic and passed along to the client in nginx, removing the current content-disposition mapping in nginx as it relies on the old way of storing files locally (with their extension).