[development] Browser settings not respected when files are private

[Darren Oh]

http://drupal.org/node/61528

Ever since it was released, the upload module has added a Content- 
Disposition header when the files are set to private. Browsers that  
understand this header use it to determine whether to download the  
file or display it in a browser window.

Users find it confusing when their browsers don't behave  
consistently. For example, PDF files display in a browser window when  
files are set to public (no Content-Disposition header), but they  
have to be saved if files are set to private (Drupal adds a "Content- 
Disposition: attachment" header).

Why don't we leave off this header so files can be handled  
consistently according to the settings users have made in their  
browsers?