Python magic is not recognizing the correct content
Use magic on the original file. 
You also need to take into account that CSV is really just a text file that uses particular characters to delimit the content. There is no explicit identifier that indicates that the file is a CSV file. Even then the CSV module needs to be configured to use the appropriate delimiters.
The delimiter specification of a CSV file is either defined by your program or needs to be configured (see importing into Excel as an example, you are presented with a number of options to configure the type of CSV to import).
I have parsed the content of a file to a variable that looks like this;

b'8,092436.csv,,20f85'

I would now like to find out what kind of filetype this data is coming from, with;

print(magic.from_buffer(str(decoded, 'utf-8'), mime=True))

This prints;

application/octet-stream

Anyone know how I would be able to get a result saying 'csv'?