add global variable for dicom file extension

mu_map/file/__init__.py

 import os
 
-from mu_map.file.dicom import load_dcm, load_dcm_img
+from mu_map.file.dicom import load_dcm, load_dcm_img, EXTENSION_DICOM
 from mu_map.file.dicom_to_interfile import to_interfile
 from mu_map.file.interfile import load_interfile, load_interfile_img, Interfile
 
 def is_dicom(filename: str):
     _, ext = os.path.splitext(filename)
-    return ext == ".ext"
+    return ext == EXTENSION_DICOM
 
 def load_as_interfile(filename: str) -> Interfile:
     if is_dicom(filename):

mu_map/file/dicom.py

@@ -22,6 +22,10 @@ Thus, when a DICOM image is updated, UIDs have to be changed for
 which the following prefix is used.
 """
 UID_PREFIX = "1.2.826.0.1.3680043.2.521."
+"""
+Default extension of dicom files.
+"""
+EXTENSION_DICOM = ".dcm"
 
 
 def load_dcm(filename: str) -> Tuple[pydicom.dataset.FileDataset, np.ndarray]: