diff --git a/openmc/plots.py b/openmc/plots.py
index df4a766331..02a44449be 100644
--- a/openmc/plots.py
+++ b/openmc/plots.py
@@ -171,8 +171,14 @@ def _get_plot_image(plot, cwd):
     from IPython.display import Image
 
     # Make sure .png file was created
-    stem = plot.filename if plot.filename is not None else f'plot_{plot.id}'
-    png_file = Path(cwd) / f'{stem}.png'
+    png_filename = plot.filename if plot.filename is not None else f'plot_{plot.id}'
+
+    # Add file extension if not already present. The C++ code added it
+    # automatically if it wasn't present.
+    if Path(png_filename).suffix != ".png":
+        png_filename += ".png"
+
+    png_file = Path(cwd) / png_filename
     if not png_file.exists():
         raise FileNotFoundError(
             f"Could not find .png image for plot {plot.id}. Your version of "
@@ -630,7 +636,7 @@ class Plot(PlotBase):
         cv.check_type('plot meshlines', meshlines, dict)
         if 'type' not in meshlines:
             msg = f'Unable to set the meshlines to "{meshlines}" which ' \
-                  'does not have a "type" key'
+                'does not have a "type" key'
             raise ValueError(msg)
 
         elif meshlines['type'] not in ['tally', 'entropy', 'ufs', 'cmfd']:
@@ -968,8 +974,13 @@ class Plot(PlotBase):
         # Run OpenMC in geometry plotting mode and produces a h5 file
         openmc.plot_geometry(False, openmc_exec, cwd)
 
-        stem = self.filename if self.filename is not None else f'plot_{self.id}'
-        h5_voxel_file = Path(cwd) / f'{stem}.h5'
+        h5_voxel_filename = self.filename if self.filename is not None else f'plot_{self.id}'
+
+        # Add file extension if not already present
+        if Path(h5_voxel_filename).suffix != ".h5":
+            h5_voxel_filename += ".h5"
+
+        h5_voxel_file = Path(cwd) / h5_voxel_filename
         if output is None:
             output = h5_voxel_file.with_suffix('.vti')