Changed the appearance of attachment text
Revamped attachment download functionality
-Remebers the download location of the attachment so that there's no
need to download the attachment everytime.
-The download progress is now shown within the app instead of the
notification bar.
-The file open intent is also within the app. The open intent in the
notification bar couldn't determine the correct MIME type 
-When downloading an attachment that already exisits (same id and
filename), the old attachment is deleted
-Added ability to delete downloaded attachment from local storage