diff --git a/large_image/constants.py b/large_image/constants.py
index fd841bea8..dbe6755fc 100644
--- a/large_image/constants.py
+++ b/large_image/constants.py
@@ -27,9 +27,10 @@ class SourcePriority(enum.IntEnum):
     LOWER = 6
     IMPLICIT_HIGH = 7
     IMPLICIT = 8
-    FALLBACK_HIGH = 9
-    FALLBACK = 10
-    MANUAL = 11  # This and higher values will never be selected automatically
+    IMPLICIT_LOW = 9
+    FALLBACK_HIGH = 10
+    FALLBACK = 11
+    MANUAL = 12  # This and higher values will never be selected automatically
 
 
 TILE_FORMAT_IMAGE = 'image'
diff --git a/sources/bioformats/large_image_source_bioformats/__init__.py b/sources/bioformats/large_image_source_bioformats/__init__.py
index a23850f76..40f504b6b 100644
--- a/sources/bioformats/large_image_source_bioformats/__init__.py
+++ b/sources/bioformats/large_image_source_bioformats/__init__.py
@@ -767,6 +767,10 @@ def addKnownExtensions(cls):
                         'fake', 'no'}:
                     if ext not in cls.extensions:
                         cls.extensions[ext] = SourcePriority.IMPLICIT
+            # These were found by reading some OMERO test files
+            for ext in {'columbusidx', 'dv_vol', 'lifext'}:
+                if ext.lower() not in cls.extensions:
+                    cls.extensions[ext.lower()] = SourcePriority.IMPLICIT_LOW
 
 
 def open(*args, **kwargs):
diff --git a/sources/gdal/large_image_source_gdal/__init__.py b/sources/gdal/large_image_source_gdal/__init__.py
index 5f7b2ae3e..3d88cd813 100644
--- a/sources/gdal/large_image_source_gdal/__init__.py
+++ b/sources/gdal/large_image_source_gdal/__init__.py
@@ -1075,6 +1075,16 @@ def addKnownExtensions(cls):
                     if drvmimes is not None:
                         if drvmimes not in cls.mimeTypes:
                             cls.mimeTypes[drvmimes] = SourcePriority.IMPLICIT
+            # This list was compiled by trying to read the test files in GDAL's
+            # repo.
+            for ext in {
+                    'adf', 'aux', 'demtif', 'dim', 'doq', 'flt', 'fst',
+                    'gdbtable', 'gsc', 'h3', 'idf', 'lan', 'los', 'lrc',
+                    'mapml', 'mint.bin', 'mtw', 'nsf', 'nws', 'on2', 'on9',
+                    'osm.pbf', 'pjg', 'prj', 'ptf', 'rasterlite', 'rdb', 'rl2',
+                    'shx', 'sos', 'tif.grd', 'til', 'vic', 'xlb'}:
+                if ext.lower() not in cls.extensions:
+                    cls.extensions[ext.lower()] = SourcePriority.IMPLICIT_LOW
 
 
 def open(*args, **kwargs):
diff --git a/sources/pil/large_image_source_pil/__init__.py b/sources/pil/large_image_source_pil/__init__.py
index b4b1ac5fc..6bae96297 100644
--- a/sources/pil/large_image_source_pil/__init__.py
+++ b/sources/pil/large_image_source_pil/__init__.py
@@ -325,6 +325,10 @@ def addKnownExtensions(cls):
             for mimeType in PIL.Image.MIME.values():
                 if mimeType not in cls.mimeTypes:
                     cls.mimeTypes[mimeType] = SourcePriority.IMPLICIT_HIGH
+            # These were found by reading various test files.
+            for ext in {'ppg'}:
+                if ext.lower() not in cls.extensions:
+                    cls.extensions[ext.lower()] = SourcePriority.IMPLICIT_LOW
 
 
 def open(*args, **kwargs):
diff --git a/sources/rasterio/large_image_source_rasterio/__init__.py b/sources/rasterio/large_image_source_rasterio/__init__.py
index 973769e10..098f561ed 100644
--- a/sources/rasterio/large_image_source_rasterio/__init__.py
+++ b/sources/rasterio/large_image_source_rasterio/__init__.py
@@ -1077,6 +1077,15 @@ def addKnownExtensions(cls):
             for ext in rasterio.drivers.raster_driver_extensions():
                 if ext not in cls.extensions:
                     cls.extensions[ext] = SourcePriority.IMPLICIT
+            # This list was compiled by trying to read the test files in GDAL's
+            # repo.
+            for ext in {
+                    'adf', 'aux', 'demtif', 'dim', 'doq', 'flt', 'fst', 'gsc',
+                    'h3', 'lan', 'los', 'lrc', 'mint.bin', 'mtw', 'nsf', 'nws',
+                    'on9', 'pjg', 'png.ovr', 'prj', 'ptf', 'rasterlite', 'rdb',
+                    'tif.grd', 'til', 'vic', 'xlb'}:
+                if ext.lower() not in cls.extensions:
+                    cls.extensions[ext.lower()] = SourcePriority.IMPLICIT_LOW
 
 
 def open(*args, **kwargs):