At this time there is no way to monitor a file using a variable for the file name. This has been requested as a feature request (TT #298 for internal folks). In your particular situation, you could workaround this by configuring IIS to log to extend.log. As a result, the current log file will always be extend.log and use a rotation scheme similar to what Apache uses.
Hope this helps.
Chris Foley - SolarWinds - Support Specialist
Support:866.530.8040 || Fax:512.857.0125
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