Trim end of name, keeping ext

Tests/test_file_im.py

@@ -1,3 +1,5 @@
+import filecmp
+
 import pytest
 from PIL import Image, ImImagePlugin
 
@@ -15,6 +17,13 @@ def test_sanity():
         assert im.format == "IM"
 
 
+def test_name_limit(tmp_path):
+    out = str(tmp_path / ("name_limit_test" * 7 + ".im"))
+    with Image.open(TEST_IM) as im:
+        im.save(out)
+    assert filecmp.cmp(out, "Tests/images/hopper_long_name.im")
+
+
 @pytest.mark.skipif(is_pypy(), reason="Requires CPython")
 def test_unclosed_file():
     def open():

src/PIL/ImImagePlugin.py

@@ -348,9 +348,14 @@ def _save(im, fp, filename):
 
     fp.write(("Image type: %s image\r\n" % image_type).encode("ascii"))
     if filename:
-        # Each line must be under length 100, or: SyntaxError("not an IM file")
+        # Each line must be 100 characters or less,
+        # or: SyntaxError("not an IM file")
+        # 8 characters are used for "Name: " and "\r\n"
         # Keep just the filename, ditch the potentially overlong path
-        fp.write(("Name: %s\r\n" % os.path.basename(filename)).encode("ascii"))
+        name, ext = os.path.splitext(os.path.basename(filename))
+        name = "".join([name[: 92 - len(ext)], ext])
+
+        fp.write(("Name: %s\r\n" % name).encode("ascii"))
     fp.write(("Image size (x*y): %d*%d\r\n" % im.size).encode("ascii"))
     fp.write(("File size (no of images): %d\r\n" % frames).encode("ascii"))
     if im.mode in ["P", "PA"]: