Index: trac/mimeview/enscript.py
===================================================================
--- trac/mimeview/enscript.py	(revision 7601)
+++ trac/mimeview/enscript.py	(working copy)
@@ -20,7 +20,7 @@
 from trac.config import Option, ListOption
 from trac.core import *
 from trac.mimeview.api import IHTMLPreviewRenderer, Mimeview
-from trac.util import NaivePopen
+from trac.util import find_executable, NaivePopen
 from trac.util.html import escape, Deuglifier
 
 __all__ = ['EnscriptRenderer']
@@ -126,9 +126,10 @@
         # Extend default MIME type to mode mappings with configured ones
         if not self._types:
             self._types = {}
-            self._types.update(types)
-            self._types.update(
-                Mimeview(self.env).configured_modes_mapping('enscript'))
+            if find_executable(self.path) != None:
+                self._types.update(types)
+                self._types.update(
+                    Mimeview(self.env).configured_modes_mapping('enscript'))
         return self._types.get(mimetype, (None, 0))[1]
 
     def render(self, context, mimetype, content, filename=None, rev=None):
Index: trac/util/__init__.py
===================================================================
--- trac/util/__init__.py	(revision 7601)
+++ trac/util/__init__.py	(working copy)
@@ -84,6 +84,38 @@
                 raise Exception('Failed to create unique name: ' + path)
             path = '%s.%d%s' % (parts[0], idx, parts[1])
 
+def find_executable(executable, path=None):
+    """Try to find 'executable' in the directories listed in 'path' (a
+    string listing directories separated by 'os.pathsep'; defaults to
+    os.environ['PATH']).  Returns the complete filename or None if not
+    found
+    """
+    if path is None:
+        path = os.environ['PATH']
+    paths = path.split(os.pathsep)
+    extlist = ['']
+    if os.name == 'os2':
+        (base, ext) = os.path.splitext(executable)
+        # executable files on OS/2 can have an arbitrary extension, but
+        # .exe is automatically appended if no dot is present in the name
+        if not ext:
+            executable = executable + ".exe"
+    elif sys.platform == 'win32':
+        pathext = os.environ['PATHEXT'].lower().split(os.pathsep)
+        (base, ext) = os.path.splitext(executable)
+        if ext.lower() not in pathext:
+            extlist = pathext
+    for ext in extlist:
+        execname = executable + ext
+        if os.path.isfile(execname):
+            return execname
+        else:
+            for p in paths:
+                f = os.path.join(p, execname)
+                if os.path.isfile(f):
+                    return f
+    else:
+        return None
 
 class NaivePopen:
     """This is a deadlock-safe version of popen that returns an object with