Why does UMN-Mapserver shows an ERDAS Image-File (.img) as white shape?

I want to render an ERDAS-Image-file (suffix .img) with the UMN-Mapserver. The data is rendered on the right position and with the correct shape, but all data is white instead of an raster-image. The Image contains many layers. My mapfile looks like this:

MAP
NAME "Test"

WEB
                  METADATA
                                         "wms_title" "test"
                                         "WMS_SRS" "epsg:31466 epsg:31467 epsg:31468  epsg:31469 epsg:4326 epsg:25832 epsg:3035"
                  END
                  LOG "test.log"
                  IMAGEPATH "."
END

SHAPEPATH "."
PROJECTION
                  "init=epsg:32632"
END

LAYER
                 NAME    "testlayer"
                 TYPE    RASTER
                 DATA    "test.img"
                 STATUS  ON
                 OFFSITE 0 0 0
END

OUTPUTFORMAT
                  NAME png
                  DRIVER "GD/PNG"
                  MIMETYPE "image/png"
                  IMAGEMODE RGBA
END

END

To give an answer to my own question: The input-file had 16 Bit per channel and that didn't worked out. The mapserver can scale the colors, but you need the data from the people, that have knowledge about the image. In my case, I was said to scale from 0-22000, so I wrote the following line to the layer-definition:

PROCESSING "SCALE=0,22000"

That worked well, now I can see structure in the image. If you don't know about the correct scale, you could try the following

PROCESSING "SCALE=AUTO"

I hope this helps someone, who runs into the same trouble in the future.