Marcel::Magic.by_magic return nil for a plain textfile

[ericproulx]

Marcel 1.0.2

File.write('/tmp/foo.txt', "hello world")
Marcel::Magic.by_magic(File.read('/tmp/foo.txt'))
# returns nil

It's been reported in 2018 in Mimemagic and I think its the same issue when looking at tika.xml

[jeremy]

There are no magic bytes in a plain text file, so nil is the appropriate response.

That's what allows the type detection heuristic to fall back to provided content type headers and filename extensions as well.

If this returned text/plain for non-binary files, that'd mis-classify virtually all non-binary files.