At first the windows explorer will search for all files that have "test" as part of the file name, but if you scroll to the bottom of the search results you can see the following:
Click on File Contents and it will perform a search within the files for the keyword "my key word".
Nevertheless, I cannot tell why this option is not available in all folders like on the Desktop.
Even for this context windows implemented a solution: Start your query with
content: and join the keyword.
Quotation marks are required if you are searching for more than one word.
As general as these solutions are supposed work, they failed in my practice test, where it should to find a keyword from a python file.