[FTS] Allow indexing & searching document attachments

Request came from (CBSE-10573)

• When users add attachments for documents, FTS interprets it as a binary blob.
• FTS to allow users to search within the attachments - based on the type of the attachment, index that content and allow users to search that content.

Here's an article that gives a picture of how elastic supports this - https://qbox.io/blog/index-attachments-files-elasticsearch-mapper