Improve PPM tests

Tests/test_file_ppm.py

@@ -1,6 +1,6 @@
 import pytest
 
-from PIL import Image
+from PIL import Image, UnidentifiedImageError
 
 from .helper import assert_image_equal, assert_image_similar, hopper
 
@@ -50,12 +50,30 @@ def test_pnm(tmp_path):
             assert_image_equal(im, reloaded)
 
 
+def test_header_with_comments(tmp_path):
+    path = str(tmp_path / "temp.ppm")
+    with open(path, "wb") as f:
+        f.write(b"P6 #comment\n#comment\r12#comment\r8\n128 #comment\n255\n")
+
+    with Image.open(path) as im:
+        assert im.size == (128, 128)
+
+
+def test_nondecimal_header(tmp_path):
+    path = str(tmp_path / "temp.djvurle")
+    with open(path, "wb") as f:
+        f.write(b"P6\n128\x00")
+
+    with pytest.raises(UnidentifiedImageError):
+        Image.open(path)
+
+
 def test_truncated_file(tmp_path):
     path = str(tmp_path / "temp.pgm")
     with open(path, "w") as f:
         f.write("P6")
 
-    with pytest.raises(ValueError):
+    with pytest.raises(UnidentifiedImageError):
         Image.open(path)
 
 
@@ -65,7 +83,7 @@ def test_neg_ppm():
     # has been removed. The default opener doesn't accept negative
     # sizes.
 
-    with pytest.raises(OSError):
+    with pytest.raises(UnidentifiedImageError):
         Image.open("Tests/images/negative_size.ppm")