diff --git a/Tests/images/bw_gradient.imt b/Tests/images/bw_gradient.imt
new file mode 100644
index 00000000000..d765cf95fac
Binary files /dev/null and b/Tests/images/bw_gradient.imt differ
diff --git a/Tests/test_file_imt.py b/Tests/test_file_imt.py
new file mode 100644
index 00000000000..f56acc42949
--- /dev/null
+++ b/Tests/test_file_imt.py
@@ -0,0 +1,19 @@
+import io
+
+import pytest
+
+from PIL import Image, ImtImagePlugin
+
+from .helper import assert_image_equal_tofile
+
+
+def test_sanity():
+    with Image.open("Tests/images/bw_gradient.imt") as im:
+        assert_image_equal_tofile(im, "Tests/images/bw_gradient.png")
+
+
+@pytest.mark.parametrize("data", (b"\n", b"\n-", b"width 1\n"))
+def test_invalid_file(data):
+    with io.BytesIO(data) as fp:
+        with pytest.raises(SyntaxError):
+            ImtImagePlugin.ImtImageFile(fp)
diff --git a/src/PIL/ImtImagePlugin.py b/src/PIL/ImtImagePlugin.py
index 5790acdaf52..dc707801274 100644
--- a/src/PIL/ImtImagePlugin.py
+++ b/src/PIL/ImtImagePlugin.py
@@ -39,15 +39,19 @@ def _open(self):
         # Quick rejection: if there's not a LF among the first
         # 100 bytes, this is (probably) not a text header.
 
-        if b"\n" not in self.fp.read(100):
+        buffer = self.fp.read(100)
+        if b"\n" not in buffer:
             raise SyntaxError("not an IM file")
-        self.fp.seek(0)
 
         xsize = ysize = 0
 
         while True:
 
-            s = self.fp.read(1)
+            if buffer:
+                s = buffer[:1]
+                buffer = buffer[1:]
+            else:
+                s = self.fp.read(1)
             if not s:
                 break
 
@@ -55,7 +59,12 @@ def _open(self):
 
                 # image data begins
                 self.tile = [
-                    ("raw", (0, 0) + self.size, self.fp.tell(), (self.mode, 0, 1))
+                    (
+                        "raw",
+                        (0, 0) + self.size,
+                        self.fp.tell() - len(buffer),
+                        (self.mode, 0, 1),
+                    )
                 ]
 
                 break
@@ -63,8 +72,11 @@ def _open(self):
             else:
 
                 # read key/value pair
-                # FIXME: dangerous, may read whole file
-                s = s + self.fp.readline()
+                if b"\n" not in buffer:
+                    buffer += self.fp.read(100)
+                lines = buffer.split(b"\n")
+                s += lines.pop(0)
+                buffer = b"\n".join(lines)
                 if len(s) == 1 or len(s) > 100:
                     break
                 if s[0] == ord(b"*"):
@@ -74,13 +86,13 @@ def _open(self):
                 if not m:
                     break
                 k, v = m.group(1, 2)
-                if k == "width":
+                if k == b"width":
                     xsize = int(v)
                     self._size = xsize, ysize
-                elif k == "height":
+                elif k == b"height":
                     ysize = int(v)
                     self._size = xsize, ysize
-                elif k == "pixel" and v == "n8":
+                elif k == b"pixel" and v == b"n8":
                     self.mode = "L"