diff --git a/bv_use_cases/tiny_morphologist/__init__.py b/bv_use_cases/tiny_morphologist/__init__.py
index 65f45c3..7fb065c 100644
--- a/bv_use_cases/tiny_morphologist/__init__.py
+++ b/bv_use_cases/tiny_morphologist/__init__.py
@@ -7,9 +7,9 @@
 
 
 class BiasCorrection(Process):
-    input: field(type_=File)
+    input: field(type_=File, extensions=('.nii',))
     strength: float = 0.8
-    output: field(type_=File, write=True)
+    output: field(type_=File, write=True, extensions=('.nii',))
 
     def execute(self, context):
         with open(self.input) as f:
@@ -24,9 +24,9 @@ def execute(self, context):
     )
 
 class FakeSPMNormalization(Process):
-    input: field(type_=File)
-    template: field(type_=File)
-    output: field(type_=File, write=True)
+    input: field(type_=File, extensions=('.nii',))
+    template: field(type_=File, extensions=('.nii',))
+    output: field(type_=File, write=True, extensions=('.nii',))
     
     requirements = {
         'fakespm': {
@@ -49,9 +49,9 @@ def execute(self, context):
             f.write(content)
 
 class AimsNormalization(Process):
-    input: field(type_=File)
+    input: field(type_=File, extensions=('.nii',))
     origin: field(type_=list[float], default_factory=lambda: [1.2, 3.4, 5.6])
-    output: field(type_=File, write=True)
+    output: field(type_=File, write=True, extensions=('.nii',))
 
     path_layout = dict(
         bids={'output': {'part': 'normalized'}},
@@ -66,9 +66,9 @@ def execute(self, context):
             f.write(content)
 
 class SplitBrain(Process):
-    input: field(type_=File)
-    right_output: field(type_=File, write=True)
-    left_output: field(type_=File, write=True)
+    input: field(type_=File, extensions=('.nii',))
+    right_output: field(type_=File, write=True, extensions=('.nii',))
+    left_output: field(type_=File, write=True, extensions=('.nii',))
 
     path_layout = dict(
         bids={'output': {'part': 'split'}},
@@ -84,8 +84,8 @@ def execute(self, context):
 
 
 class ProcessHemisphere(Process):
-    input: field(type_=File)
-    output: field(type_=File, write=True)
+    input: field(type_=File, extensions=('.nii',))
+    output: field(type_=File, write=True, extensions=('.nii',))
 
     def execute(self, context):
         with open(self.input) as f: