V7 Windows or Mac - If you go to File, Utilities, Rebuild Search Index and look at Statistics, what it says for number of Internal Attachments will be be different that what Statistics said before rebuilding the search index. Then if you go to Reports and run the No Attachments Report you can see Thoughts listed that actually have attachments in them.
(I have the brain set to move attachments into thoughts, not link to them when dragged in to a thought)
To make it easy to see create a new brain and add three thoughts to it.
Drag an Excel file into one thought and a pdf into another thought.
Statistics will show 2 Internal Attachments.
Run Rebuild Search Index and when it's done Statistics will show 1 Internal Attachment. Run the No Attachments Report and you'll see the thought that has an attachment but is counted as not having one.