Custom File Fields break Attachment view.

Activities with an uploaded file do not work.

This was discovered on dmaster.demo.civicrm.org during follow-up to CRM-16869 to verify file uploads to Petition Signatures, however is not restricted to that Activity type.

Steps to Reproduce:

1. Create a Custom data group for Activities including a "File" type field
2. Create a Profile including the "File" field.
3. Create a Petition using the profile above for the Activity profile.
4. Sign the petition, including a file upload

Alternatively

1. Create Custom data group as above.
2. Use "Contacts > New Activity" to create an Activity of any available type
3. Attach a file to the activity via the custom field.

Results:
A new Activity is created including the uploaded file, which can be discovered through the API including details about the uploaded file.
Trying to view the Activity through the user interface (e.g. http://dmaster.demo.civicrm.org/civicrm/activity?action=view&reset=1&id=627&atype=65&cid=202&context=home ) results fails with the error message:

Sorry but we are not able to provide this at the moment.
One of parameters (value: foo_9c2815bf894c2d6f47efe7297d0490ff.txt) is not of the type Integer