Index: email/message.py
===================================================================
--- email/message.py	(revision 65356)
+++ email/message.py	(working copy)
@@ -24,6 +24,7 @@
 # simple.  It isn't strictly RFC 2045 (section 5.1) compliant, but it catches
 # most headers found in the wild.  We may eventually need a full fledged
 # parser eventually.
+# [see: _fast_paramre_name]
 paramre = re.compile(r'\s*;\s*')
 # Regular expression that matches `special' characters in parameters, the
 # existance of which force quoting of the parameter value.
@@ -82,6 +83,18 @@
     else:
         return utils.unquote(value)
 
+def _fast_paramre_name(txt):
+    """ paramre has exponential bad behavior for strings with large amounts
+        of whitespace (issue2676).  This function performs the same
+        operation that paramre was used for but linearly.
+        paramre is still included in the module for backwards compatibility.
+    """
+    semi_ind = txt.find(';')
+    if semi_ind >= 0:
+        ctype = txt[:semi_ind]
+    else:
+        ctype = txt
+    return ctype.strip().lower()
 
 
 class Message:
@@ -443,7 +456,7 @@
         if value is missing:
             # This should have no parameters
             return self.get_default_type()
-        ctype = paramre.split(value)[0].lower().strip()
+        ctype = _fast_paramre_name(value)
         # RFC 2045, section 5.2 says if its invalid, use text/plain
         if ctype.count('/') != 1:
             return 'text/plain'