custom content-type support

[LinusU]

The tests could be a bit more extensive and I haven't added documentation yet, but this does all the heavy lifting.

I've replaced s3fs with the official aws-sdk so this should close #3 as well as #7.

Use it like so:

const multerS3 = require('multer-s3')
const storage = new multerS3({
  ...
  contentType: multerS3.AUTO_CONTENT_TYPE
})

contentType can also be your own function with the signature (req, file, cb).

[anacronw]

Elegant refactor, I expected there'd be more code involved w/ this feature

[anacronw]

Can you add some quick documentation to the README? Everything else looks good to me

[anacronw]

@ch0p57ickz thanks for testing

[LinusU]

Thank you for landing, we should still fix your nits and add documentation though :)

[Mobasher-NetLinks]

How we can use it inside shouldTransform to check if its valid type or no, I mean checking with magic number, I used it inside shouldTransform but it stuck on that point, and not goes to tranform function

[thenengah]

Would be nice to update the docs. Let me know if I can help out!