xls files need 521 bytes

[vgambier-altus]

The documentation says to pass the first 261/262 bytes. However, after some testing, it seems that for .xls files, 261 bytes is not enough. Passing 520 bytes is not enough either. Passing 521 bytes is enough, and .xls files are correctly identified as .xls files.

This issue does not affect .xlsx files, which are correctly identified with only 261 bytes.

I'm not sure if this is a documentation issue or an implementation issue.

[CatKasha]

readme is outdated, filetype.py reads 8192 bytes and should detect xls and xlsx files
b71bf17