This is a frequent and common question that every SharePoint/InfoPath developer come across at some point of time. The fact is attached files in InfoPath forms are not indexed. When you upload a file in an InfoPath form using the File Attachment control, InfoPath saves not only the binary data, but in a binary way extra information like name etc. When SharePoint Search is indexing, it finds the InfoPath xml file, it can read inside, but then the indexer finds a string of strange characters that the file attachment control has encoded and eventually the attached file content is not indexed.
The other work around would be to upload the file in a document library and providing the link in InfoPath form.
You may find some blog over web saying that SharePoint does index file attachments. But this is not true and not confirmed by Microsoft. But rather I have confirm from Microsoft experts that SharePoint does not indexes attachments in InfoPath form.
You may check this http://sharepointbeeeye.com/infopath-2010-a-better-file-attachment-solution/ article. It says it provides a better file attachment solution in InfoPath. I haven't tried this solution myself but you can try and let me know.
