Purpose

Atlassian Companion, which is used for editing files in Confluence, blocks users from editing files that may pose a security risk, such as executable files.  Companion will not open any file with a COM, .EXE, .BAT, or .CMD file extension.  Companion will also refuse to open any file that has no default application associated with it, and has a file extension listed in the PATHEXT environment variable. The default value of this environment variable is:

.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC

You can further limit the files that companion is allowed to open in Windows by setting the COMPANION_BLOCK_LIST  environment variable.  This change must be made on each individual user's computer. There's no way to set this for all users in your site.  This environment variable is only available in Atlassian Companion 1.0.0 and later. 

Solution

To set the COMPANION_BLOCK_LIST environment variable:

1. In Windows settings, search for Environment Variables and choose either Edit the system environment variables or Edit the environment variables for your account. 
2. Click the Environment Variables button.
3. Click New'.

4. In the Variable name field, enter 'COMPANION_BLOCK_LIST'.
   In the Variable value field, enter the file extensions you wish to block, separated by a semicolon as follows. In this example we're blocking Photoshop and Visio files. 

   .PSD;.VSD

5. Restart Atlassian Companion app.