completely separate the request filtering from the column filtering. In other words, the requests should appear based on (currently non-existent) 'input filters', which could have any number of filter conditions added, and a capability to be set as the default filters. The grid column filters would apply only to rows that actually appear in the grid (the requests that pass the input filters).
Since that won't happen in the short term, there are a couple of things that would help a little with the current approach. The first one is the addition of a 'Does not end with' filter option. The second thing would be to allow the use of a token to separate multiple values for the criteria. In other words, I could say 'Ends with', and have the value be a string like ".css:.js:.ico" (that's hard to see here - it's a semi-colon separated list of file extensions) The code could then operate on each value in the list.