How to support .dcm image format file for uploading in an article?
Unfortunately, it is not possible to allow image extension that cannot commonly displayed by a browser. You would have to use a filefield instead. 

This has been discussed at:

https://drupal.org/node/1081060

https://drupal.org/node/1014816

And for D6 at:

https://drupal.org/node/515152