Sometimes attachments cannot be uploaded in ITSM 6.x / 7.x ; this can happen 1 out of 3 tries.
The upload aborts with the error message: "Attachment Upload Failed - Error 404 - not found". The Smart Client is no longer responding.
Configure double escaping
To configure double escaping, use the following syntax:
1. Open a CMD prompt as an Administrator
2. Type CD C:\Windows\System32\inetsrv
3. Appcmd set config /section:requestfiltering /allowdoubleescaping:true
4. Perform an IISreset
See also http://www.iis.net/learn/manage/configuring-security/configure-request-filtering-in-iis