add step for decryption of downloaded data from DUMA - web interface

external/exchange-channels/duma.md

@@ -59,6 +59,16 @@ In the following steps we provide instructions on how to use DUMA web interface.
 5. At any time you can launch **IBM Aspera Connect** to display the status of uploads to or downloads from your computer. <br/>
   ![Alt](duma_8.png "Title") 
 
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+6. All data are encrypted on server side and they stay encrypted also upon download. For decryption, you have to navigate into your **IBM Aspera Connect** window and click "**Unlock encrypted files**". <br/>
+  ![Alt](duma_9.png "Title") <br/>
+You will be prompted for encryption passphrase which you should already have received from the repository administrator. <br/>
+  ![Alt](duma_10.png "Title") <br/>
+Encrypted files are by default kept on your disc after decryption. If you want to change this behaviour, navigate to Options->Settings and check "Delete encrypted files when finished" box. 
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 <a name="DUMA_CLI"></a>
 ## Accessing DUMA via Command-Line Tool
 In the following steps we describe instructions on how to use DUMA web interface.