Searching files by content

[s0i37]

Hello.
How can I use Invoke-FileFinder or Find-InterestingFile for searching files by content.
It seems that these commandlets could search only by file name.
Very offten users hide your passwords in files like 123.txt which couldn't be found with this approach.
Moreover users like very much to store files in doc/docx-files, which couldn't be grepped.
Thank you.