This seems to be a regression of this: https://jira.atlassian.com/browse/CONF-12006

Steps to Reproduce

1. Setup Confluence 5.5.4 and 5.6.3
2. Attach this file to any page (taken from CONF-12006)
3. Wait for the index flush to run, or you can trigger it via Confluence Admin >> Scheduled Jobs, or you can rebuild the index via Confluence Admin >> Content Indexing
4. Search for "random string of words"

Notice that the attachment will be found in Confluence 5.3, but not 5.5 and 5.6

Workaround

Edit the document and save it as .doc instead. Then reattach to Confluence. .doc files can be indexed, but not .docx, it seems...