Problem

User seeing the following error when downloading an attachment

Diagnosis

Environment

• User using IIS
  
  

Example of File extensions

• .cs
• .csproj
• .sitemap
• .mdf
• .config

Cause

This problem appears if you are using IIS as a proxy server in front of Confluence and the file you are trying to download contains a specific "filtered" extension. By default, IIS contains a filter named "Request Filtering" which prohibit access to specific files and folders.

Workaround

If the file contains an extension in the list above, it's possible to allow it for download executing the following commands:

1. To allow downloading a file present in the in the file extensions list (i.e. '.config')

   1. Open an administrator command prompt on the server hosting IIS
   
   2. Execute the following command 
   
   appcmd set config "Web app name" /section:requestFiltering
   /-fileExtensions.[fileextension='.config']

2. To allow downloading a file present in the hidden segments list (i.e. 'web.config')

   1. open an administrator command prompt on the server hosting IIS
   2. execute the following command
   
    appcmd set config "Web app name" /section:requestFiltering
   /-hiddenSegments.[segment='web.config']