Modify files before storing them

It would be useful to have the ability to modify files as they are uploaded. A simple example would be resizing an image using gd.

My initial thoughts are the fileFilter function could return the file as the second argument rather than a boolean. That way it could be manipulated in the fileFilter function and then be passed on to be stored, or dropped if false was passed instead of the file.

https://github.com/expressjs/multer/blob/master/lib/make-middleware.js#L94

This would be more useful when using an external storage engine such as S3 as you wouldn’t have to upload -> download -> edit -> re-upload.

[LinusU]

@wesleytodd Maybe it’s better to make the transformFile option belong on the global multer instance? That way we don’t have to implement it in every storage module.

Also, I think that transformFile should be a function that takes (req, file, cb) that calls the cb with a replacement stream

[jonchurch]

Current PR that would allow this is https://github.com/expressjs/multer/pull/399