It's useful to be able to combine kNN with a filter ("find the nearest documents that also satisfy this search query"). We could extend the recently added _knn_search endpoint to accept search filters too:

POST index/_knn_search
{
 "knn": {
  "field": "image_vector",
  "query_vector": [0.3f, 0.1f, ...],
  "k": 10,
  "num_candidates": 100
 },
 "filter": {
  "term": {
   "file_type": "jpeg"
  }
 }
}

The filter section accepts a search filter, defined through the Query DSL. The response will contain the nearest k documents that also match the filter. We could extend the filter section to accept a list of filters if this proves helpful.