Use DetectContentType about codecs HOT 4 CLOSED

there are two things
that DetectContentType takes the actual data to decide the content type
if the Content-Type header is set, we should use that mime package to get it from there

from codecs.

we can use DetectContentType if there's no Content-Type header

from codecs.

So:

1. If Content-Type header is set, use the mime package to parse it properly.
2. If Content-Type header is not set, use DetectContentType to parse it
   a. How will we get the content type in that case? Where does it come from?

from codecs.

This is being solved in #22.

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