diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
index 8b630ce3e442..d556d74edcce 100644
--- a/lib/matplotlib/image.py
+++ b/lib/matplotlib/image.py
@@ -1351,9 +1351,9 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
     from matplotlib.figure import Figure
 
     # Fast path for saving to PNG
-    if (format == 'png' or format is None or
-            isinstance(fname, six.string_types) and
-            fname.lower().endswith('.png')):
+    if (format == 'png' or (format is None and
+                            isinstance(fname, six.string_types) and
+                            fname.lower().endswith('.png'))):
         image = AxesImage(None, cmap=cmap, origin=origin)
         image.set_data(arr)
         image.set_clim(vmin, vmax)
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
index 9dc83efe27dc..e56ac4cfbe93 100644
--- a/lib/matplotlib/tests/test_image.py
+++ b/lib/matplotlib/tests/test_image.py
@@ -176,6 +176,17 @@ def test_imsave_color_alpha():
 
         assert_array_equal(data, arr_buf)
 
+def test_imsave_format(tmpdir):
+    # Test imsave output format based on the input filename
+    # (i.e. when the "format" keyword is None).
+
+    filename = str(tmpdir.join('test_imsave.svg'))
+    plt.imsave(filename, np.zeros((10, 10)))
+
+    lines = open(filename).readlines()
+    assert lines[0].startswith('<?xml')
+    assert lines[1].startswith('<!DOCTYPE svg')
+
 @image_comparison(baseline_images=['image_alpha'], remove_text=True)
 def test_image_alpha():
     plt.figure()