Symptom:

You receive the following error when attempt to change the mime type for .manifest files to "text/cache-manifest" for your domain. 

"Operation failed" - "The operation you were performing failed. You can retry the operation with or without changing its parameters. You may also want to report this problem to our support so that we could help you as soon as possible."

Resolution:

This indicates that the mime type is already defined server-wide, likely set to the server default of "applicatoin/x-ms-manifest" type.  We can make your site not inherit the server wide setting, and define this as a local mime type for just your domain.  Simply submit a support request for this.  You can use the following request template below.

As discussed in KB article 12345, please make the following change in IIS for me, remove the inherited mime type definition of "application/x-ms-manifest" and create a local mime type definition of "text/cache-manifest" for the following domains:  _____________________