diff --git a/src/documents/mail.py b/src/documents/mail.py
index d54b387b7..f43f6604c 100644
--- a/src/documents/mail.py
+++ b/src/documents/mail.py
@@ -130,7 +130,7 @@ def file_name(self):
 class Attachment(object):
 
     SAFE_SUFFIX_REGEX = re.compile(
-        r"^(application/(pdf))|(image/(png|jpeg|gif|tiff))$")
+        r"^(application/(?:x-)?(pdf))|(image/(png|jpeg|gif|tiff))$")
 
     def __init__(self, data, content_type):