image_test.py: fix TestBlockedFileTypes accidentally blocking csh files

TestBlockedFileTypes accidentally blocks csh files because regex text/c
will match text/csh. This CL fixes it.

BUG=b:175095747
TEST=CQ

Change-Id: I5faeb98c264b493461d090fecf04372377a7a3bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2578801
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Boleyn Su <boleynsu@chromium.org>
Tested-by: Boleyn Su <boleynsu@chromium.org>
Auto-Submit: Boleyn Su <boleynsu@chromium.org>
diff --git a/cros/test/image_test.py b/cros/test/image_test.py
index 8fecee7..4e6bb79 100644
--- a/cros/test/image_test.py
+++ b/cros/test/image_test.py
@@ -113,8 +113,8 @@
     The allow list has higher precedence than the block list.
     """
     blocked_patterns = [re.compile(x) for x in [
-        r'text/x-c\+\+',
-        r'text/x-c',
+        r'^text/x-c\+\+$',
+        r'^text/x-c$',
     ]]
     allowed_patterns = [re.compile(x) for x in [
         r'.*/braille/.*',