How to reproduce:
#Load a DICOM Seg (e.g. a one just generated and stored as DICOM Seg)
#Try to store it as DICOM Seg -> Not possible. Data format DCM is not provided for selection.
See also {T26827} and {T26820} which (very likely) ran into the problem.

Reason:
The DicomSegIO checks for certain Properties to deduce the confindence level. One property is a StringLookUp names "referenceFiles" that contains the pathes to the source/image files. If the property is not there the IO does not support the data -> no ".dcm" to choose. And it is not there because this very information is not stored any more in the DICOM SEG.

Solution:
The best solution would be to get rid of "refernceFiles" at all (as it also was discussed by @metzger in T25192) and that in a sound way.

1. Do T26951: DICOM SEG - DicomSegIO should just use properties of the passed segmentation object
2. Ensure that the DicomSegIO load function populates the segmentation object with all informations needed, that can be directly extracted from the DICOM file if not passed through by DCMQI.