When you place a loader next to an inventory it immediately starts extracting items,
but if you need it to be filtered your options are limited:

1 - place it next to the container without a belt attached, set the filter, toggle it to insert & then extract again.
2 - place it away from the container, then cut paste it as a blueprint and place it preconfigured next to the container.

This mod helps with the first one: if you set a filter on a loader it'll try to put all the items currently occupying the loader tile back into the container from wince it came (or spill them on the ground), no need to manually flip it twice.

(note: this mod currently does not clear out any conveyors the loader is outputting into, only items inside the loader)