In plone how can I make an uploaded file as NOT downloadable?
Cue tune from Hotel California:  "You can check out any time you like, but you can never leave." 
You do not not really want to disable all downloading, I believe that you really just want to disable downloads from all users but Owner.  There is no practical use for putting files into something with no vehicle for EVER getting them back out...
...so you need to solve this problem with workflow:

Use a custom workflow definition that has a state for this behavior ("Confidential").  Ensure that "View" permission is not inherited from folder above in the permissions for this state, and check "Owner" (and possibly "Manager" if you see fit) as having "View" permission.
Set the confidential state as the default state for files.  You can do this using Workflow policy support ("placeful workflows") in parts of the site if you do not wish to do this site-wide.

Should you wish to make the existence of the items viewable, but the download not, you are best advised to create a custom permission and a custom type to protect downloading with a permission other than "View" (but you still should use workflow state as permission-to-role mapping templates).
I wish to make the uploaded file contents only viewable on the browser i.e using atreal.richfile.preview for doc/xls/pdf files. The file should not be downloadable at any cost. How do I remove the hyperlink for the template in a particular folder for all the files in that folder? I use Plone 4.1 There is AT at_download.