Add possibility to check only part of the file

[Lorenzo Tanganelli]

Steps to reproduce

Hi team,
I'm expecting that in the files_antivirus options, should be resent an option to scan only a part of the file. Image to have 50GB of file, it's very timeconsuming and resource consuming scan whole file, so I'm thinking to propose a "middle" solution.

I will send you a PR that enable new option that allow us to specify how much bytes we want to check, instead whole file.
if we set to -1 whole file is checked (and each chunks.

The default value will be -1

Let me know if this is good for you.
Thanks