I don't know about a filter but I know when using camera raw in
photoshop to open a raw image, you can alter the contrast which results
in more detail in shadows and (provided it is there in the first place)
highlights. It makes for a very flat looking picture but one which can
have it's attenuation increased with other tools to liven up the image.

My experience has been that under exposing when bright objects are
present by up to 2 stops allows you to use Photoshop tools the lighten
the dark areas while keeping the highlights under control. This also
results in flat looking pictures.

I think that until the sensors can capture a wider range of detail -
maybe closer to human vision - no digital camera and no filter will
produce a pleasant looking picture with the full range of such scenes
as would normally have blown highlights and blacked shadows.

Maddy