PDF file detected as non-binary

I am feeding a PDF file to `typecode.contenttype.is_binary`. As PDF files are usually considered as binary files, I would have expected the file to be detected as binary, but apparently the first bytes used for detection are looking like plain-text, leading to a wrong classification.

Example file: [antartica-3427135_640_1_libtiff.pdf](https://github.com/user-attachments/files/16411602/antartica-3427135_640_1_libtiff.pdf)