feat/fix: Improved or customizable filetype detection for actix-files

actix_files [currently detects mimetypes using mime_guess](https://github.com/actix/actix-web/blob/c6249cc00c757083ae1b29a3c4e1748185f43071/actix-files/src/named.rs#L107-L125), which [explicitly](https://docs.rs/mime_guess/2.0.5/mime_guess/) reads only the file extension to determine the type. actix-files then uses a small set of types to determine the disposition.

The issue: common developer file types (Dockerfile, Cargo.lock, more) aren't recognized by mime_guess, so the `disposition: attachment` causes browsers to download them instead of display them.

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There are several possible fixes/improvements, depending on how you look at it:

- zero config: A simple win (with a likely small perf impact) would be to additionally analyze files that `mime_guess` doesn't recognize (and which are therefore labeled `application/octet-stream`) using the git algorithm (load the first 8KB, check if that 8KB both has zero NULL bytes and is valid UTF-8) and if it appears to be text, change the mime type back to `text/plain` and `content-disposition: inline`.
- add a config or policy argument: adding another which allows users to provide a callback or something which would allow them to take this perf on themselves (and additionally for example state that type `application/pdf` should be inline) would be more flexible at the cost of larger API surface.

Would you be open to a PR implementing either or both of those? Or do you have other ideas for ways to improve the situation?