Detect JSON / PDF specific mime type

[Matteo Alessandroni]

I'm using Apache Tika to detect a file Mime Type from its base64 
rapresentation. 
Unfortunately I don't have other info about the file (e.g. extension).

Is there anything I can do to make Tika be more specific?
I'm currently using this:

Tika tika = new Tika();
tika.setMaxStringLength(-1);
String mimetype = tika.detect(Base64.decode(fileString));

and it gives me "text/plain" for JSON and PDF files, but I would like to obtain 
a more specific information: "application/json", "application/pdf" etc...

Can you help me with that?

Thanks.