At the top of the Thunderbird window, click the Tools menu and select Options, or click the menu button menu and choose Options.
   Select the Attachments panel and choose the Outgoing tab.
   Click the Add button. The Set up Filelink panel will open.
   Select the desired service provider from the drop-down list.
       Choose Box, click Set up Account, or click the Get a Box account… link to create a new account.

When I read this- Configuring Filelink- I am unable to select attachments. It is on the list but not available. To use Filelink, you must first create a new account (or configure an existing account) with an online storage service provider. At the top of the Thunderbird window, click the Tools menu and select Options, or click the menu button menu and choose Options. Select the Attachments panel and choose the Outgoing tab. Click the Add button. The Set up Filelink panel will open. Select the desired service provider from the drop-down list. Choose Box, click Set up Account, or click the Get a Box account… link to create a new account.