Add TIFFTAG_SAMPLEFORMAT to blocklist

Tests/test_file_tiff.py

@@ -4,7 +4,7 @@
 
 from .helper import unittest, PillowTestCase, hopper
 
-from PIL import Image, TiffImagePlugin
+from PIL import Image, TiffImagePlugin, features
 from PIL._util import py3
 from PIL.TiffImagePlugin import X_RESOLUTION, Y_RESOLUTION, RESOLUTION_UNIT
 
@@ -587,6 +587,30 @@ def test_close_on_load_nonexclusive(self):
             im.load()
             self.assertFalse(fp.closed)
 
+    @unittest.skipUnless(features.check("libtiff"), "libtiff not installed")
+    def test_sampleformat_not_corrupted(self):
+        # Assert that a TIFF image with SampleFormat=UINT tag is not corrupted
+        # when saving to a new file.
+        # Pillow 6.0 fails with "OSError: cannot identify image file".
+        import base64
+
+        tiff = BytesIO(
+            base64.b64decode(
+                b"SUkqAAgAAAAPAP4ABAABAAAAAAAAAAABBAABAAAAAQAAAAEBBAABAAAAAQAA"
+                b"AAIBAwADAAAAwgAAAAMBAwABAAAACAAAAAYBAwABAAAAAgAAABEBBAABAAAA"
+                b"4AAAABUBAwABAAAAAwAAABYBBAABAAAAAQAAABcBBAABAAAACwAAABoBBQAB"
+                b"AAAAyAAAABsBBQABAAAA0AAAABwBAwABAAAAAQAAACgBAwABAAAAAQAAAFMB"
+                b"AwADAAAA2AAAAAAAAAAIAAgACAABAAAAAQAAAAEAAAABAAAAAQABAAEAAAB4"
+                b"nGNgYAAAAAMAAQ=="
+            )
+        )
+        out = BytesIO()
+        with Image.open(tiff) as im:
+            im.save(out, format="tiff")
+        out.seek(0)
+        with Image.open(out) as im:
+            im.load()
+
 
 @unittest.skipUnless(sys.platform.startswith("win32"), "Windows only")
 class TestFileTiffW32(PillowTestCase):

src/PIL/TiffImagePlugin.py

@@ -1540,7 +1540,7 @@ def _save(im, fp, filename):
 
         # STRIPOFFSETS and STRIPBYTECOUNTS are added by the library
         # based on the data in the strip.
-        blocklist = [STRIPOFFSETS, STRIPBYTECOUNTS]
+        blocklist = [STRIPOFFSETS, STRIPBYTECOUNTS, SAMPLEFORMAT]
         atts = {}
         # bits per sample is a single short in the tiff directory, not a list.
         atts[BITSPERSAMPLE] = bits[0]