It only allows the extensions it's capable of reliably processing. The File gallery has a parameter to support whatever extensions you want, but it doesn't process image files. Only jpg, jpeg, gif, and png are supported by the photo gallery. You can modify the extension array of getExtensions in the below file, but it may or may not work.

components/com_comprofiler/plugin/user/plug_cbgallery/cbgallery.php

It does not support DCM extension. It supports only jpg, jpeg, gif, and png. It has zero testing for DCM extensions. Trying to force it by altering the extension validation doesn't mean it's going to actually work as it has not been tested. Reviewing Imagine library source it only will allow jpeg, jpg, gif, png, wbmp, and xbm even if using Imagick; it can not reliably process other extensions. CB Gallery it self limits to jpg, jpeg, gif, and png. The image format you're wanting to use will not work with the Photo gallery and recommend using the File gallery to just directly share the file. Your alternative is to process them to JPG or PNG before uploading to the Photo gallery.

With that said it does look like there's a missing global bug. Please make the below change and it should use the correct image software, but doesn't mean DCM will work.

IN: components/com_comprofiler/plugin/user/plug_cbgallery/cbgallery.php
ON: Line 467
FROM:TO:

Click the download button instead of the title of the file, which trys to open it. Regardless the behavior is left up to the browser. Enable debug mode and maximum error reporting then try again to expose any errors if you continue to get a blank page.