diff --git a/tracker/attachment_helpers.py b/tracker/attachment_helpers.py
new file mode 100644
index 0000000..9ed9a7c
--- /dev/null
+++ b/tracker/attachment_helpers.py
@@ -0,0 +1,112 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+
+"""Functions to help display attachments and compute quotas."""
+from __future__ import print_function
+from __future__ import division
+from __future__ import absolute_import
+
+import base64
+import hmac
+import logging
+
+from framework import urls
+from services import secrets_svc
+from tracker import tracker_helpers
+
+
+VIEWABLE_IMAGE_TYPES = [
+    'image/jpeg', 'image/gif', 'image/png', 'image/x-png', 'image/webp',
+    ]
+VIEWABLE_VIDEO_TYPES = [
+    'video/ogg', 'video/mp4', 'video/mpg', 'video/mpeg', 'video/webm',
+    'video/quicktime',
+    ]
+MAX_PREVIEW_FILESIZE = 15 * 1024 * 1024  # 15 MB
+
+
+def IsViewableImage(mimetype_charset, filesize):
+  """Return true if we can safely display such an image in the browser.
+
+  Args:
+    mimetype_charset: string with the mimetype string that we got back
+        from the 'file' command.  It may have just the mimetype, or it
+        may have 'foo/bar; charset=baz'.
+    filesize: int length of the file in bytes.
+
+  Returns:
+    True iff we should allow the user to view a thumbnail or safe version
+    of the image in the browser.  False if this might not be safe to view,
+    in which case we only offer a download link.
+  """
+  mimetype = mimetype_charset.split(';', 1)[0]
+  return (mimetype in VIEWABLE_IMAGE_TYPES and
+          filesize < MAX_PREVIEW_FILESIZE)
+
+
+def IsViewableVideo(mimetype_charset, filesize):
+  """Return true if we can safely display such a video in the browser.
+
+  Args:
+    mimetype_charset: string with the mimetype string that we got back
+        from the 'file' command.  It may have just the mimetype, or it
+        may have 'foo/bar; charset=baz'.
+    filesize: int length of the file in bytes.
+
+  Returns:
+    True iff we should allow the user to watch the video in the page.
+  """
+  mimetype = mimetype_charset.split(';', 1)[0]
+  return (mimetype in VIEWABLE_VIDEO_TYPES and
+          filesize < MAX_PREVIEW_FILESIZE)
+
+
+def IsViewableText(mimetype, filesize):
+  """Return true if we can safely display such a file as escaped text."""
+  return (mimetype.startswith('text/') and
+          filesize < MAX_PREVIEW_FILESIZE)
+
+
+def SignAttachmentID(aid):
+  """One-way hash of attachment ID to make it harder for people to scan."""
+  digester = hmac.new(secrets_svc.GetXSRFKey())
+  digester.update(str(aid))
+  return base64.urlsafe_b64encode(digester.digest())
+
+
+def GetDownloadURL(attachment_id):
+  """Return a relative URL to download an attachment to local disk."""
+  return 'attachment?aid=%s&signed_aid=%s' % (
+        attachment_id, SignAttachmentID(attachment_id))
+
+
+def GetViewURL(attach, download_url, project_name):
+  """Return a relative URL to view an attachment in the browser."""
+  if IsViewableImage(attach.mimetype, attach.filesize):
+    return download_url + '&inline=1'
+  elif IsViewableVideo(attach.mimetype, attach.filesize):
+    return download_url + '&inline=1'
+  elif IsViewableText(attach.mimetype, attach.filesize):
+    return tracker_helpers.FormatRelativeIssueURL(
+        project_name, urls.ISSUE_ATTACHMENT_TEXT,
+        aid=attach.attachment_id)
+  else:
+    return None
+
+
+def GetThumbnailURL(attach, download_url):
+  """Return a relative URL to view an attachment thumbnail."""
+  if IsViewableImage(attach.mimetype, attach.filesize):
+    return download_url + '&inline=1&thumb=1'
+  else:
+    return None
+
+
+def GetVideoURL(attach, download_url):
+  """Return a relative URL to view an attachment thumbnail."""
+  if IsViewableVideo(attach.mimetype, attach.filesize):
+    return download_url + '&inline=1'
+  else:
+    return None