diff --git a/src/calibre/utils/imghdr.py b/src/calibre/utils/imghdr.py
index 78e4a7efe0..eb35b7177b 100644
--- a/src/calibre/utils/imghdr.py
+++ b/src/calibre/utils/imghdr.py
@@ -90,10 +90,8 @@ def identify(src):
 # Subroutines per image file type #
 # ---------------------------------#
 
-tests = []
 
-
-def test_jpeg(h):
+def test_for_jpeg(h):
     """JPEG data in JFIF format (Changed by Kovid to mimic the file utility,
     the original code was failing with some jpegs that included ICC_PROFILE
     data, for example: http://nationalpostnews.files.wordpress.com/2013/03/budget.jpeg?w=300&h=1571)"""
@@ -146,116 +144,89 @@ def jpeg_dimensions(stream):
 
     return -1, -1
 
-tests.append(test_jpeg)
 
-
-def test_png(h):
+def test_for_png(h):
     if h[:8] == b"\211PNG\r\n\032\n":
         return 'png'
 
-tests.append(test_png)
 
-
-def test_gif(h):
+def test_for_gif(h):
     """GIF ('87 and '89 variants)"""
     if h[:6] in (b'GIF87a', b'GIF89a'):
         return 'gif'
 
-tests.append(test_gif)
 
-
-def test_tiff(h):
+def test_for_tiff(h):
     """TIFF (can be in Motorola or Intel byte order)"""
     if h[:2] in (b'MM', b'II'):
         if h[2:4] == b'\xbc\x01':
             return 'jxr'
         return 'tiff'
 
-tests.append(test_tiff)
 
-
-def test_webp(h):
+def test_for_webp(h):
     if h[:4] == b'RIFF' and h[8:12] == b'WEBP':
         return 'webp'
 
-tests.append(test_webp)
 
-
-def test_rgb(h):
+def test_for_rgb(h):
     """SGI image library"""
     if h[:2] == b'\001\332':
         return 'rgb'
 
-tests.append(test_rgb)
 
-
-def test_pbm(h):
+def test_for_pbm(h):
     """PBM (portable bitmap)"""
     if len(h) >= 3 and \
         h[0] == b'P' and h[1] in b'14' and h[2] in b' \t\n\r':
         return 'pbm'
 
-tests.append(test_pbm)
 
-
-def test_pgm(h):
+def test_for_pgm(h):
     """PGM (portable graymap)"""
     if len(h) >= 3 and \
         h[0] == b'P' and h[1] in b'25' and h[2] in b' \t\n\r':
         return 'pgm'
 
-tests.append(test_pgm)
 
-
-def test_ppm(h):
+def test_for_ppm(h):
     """PPM (portable pixmap)"""
     if len(h) >= 3 and \
         h[0] == b'P' and h[1] in b'36' and h[2] in b' \t\n\r':
         return 'ppm'
 
-tests.append(test_ppm)
 
-
-def test_rast(h):
+def test_for_rast(h):
     """Sun raster file"""
     if h[:4] == b'\x59\xA6\x6A\x95':
         return 'rast'
 
-tests.append(test_rast)
 
-
-def test_xbm(h):
+def test_for_xbm(h):
     """X bitmap (X10 or X11)"""
     s = b'#define '
     if h[:len(s)] == s:
         return 'xbm'
 
-tests.append(test_xbm)
 
-
-def test_bmp(h):
+def test_for_bmp(h):
     if h[:2] == b'BM':
         return 'bmp'
 
-tests.append(test_bmp)
 
-
-def test_emf(h):
+def test_for_emf(h):
     if h[:4] == b'\x01\0\0\0' and h[40:44] == b' EMF':
         return 'emf'
 
-tests.append(test_emf)
 
-
-def test_jpeg2000(h):
+def test_for_jpeg2000(h):
     if h[:12] == b'\x00\x00\x00\x0cjP  \r\n\x87\n':
         return 'jpeg2000'
 
-tests.append(test_jpeg2000)
 
-
-def test_svg(h):
+def test_for_svg(h):
     if h[:4] == b'<svg' or (h[:2] == b'<?' and h[2:5].tobytes().lower() == b'xml' and b'<svg' in h.tobytes()):
         return 'svg'
 
-tests.append(test_svg)
+
+tests = tuple(v for k, v in globals().iteritems() if callable(v) and k.startswith('test_for_'))