Allow to customize the key generation used in the companion app

[RobertAudi]

Right now the generated key only preserves the original file extension, it replaces everything before by a random string:

https://github.com/janko/uppy-s3_multipart/blob/404c511859b4f3755b0274dc5f0b29f6e65c2644/lib/uppy/s3_multipart/app.rb#L42-L45

It is possible to specify multiple prefixes but it can only be done once when the app is mounted and can't be changed at runtime.

One use case is to be able to upload files in "a directory tree" defined by Uppy. For example:

audits/             <- The name of a resource
├── 42/             <- The ID of a specific resource
│  ├── users/       <- The category of uploaded files
│  │  ├── ACME\ Europe___<RANDOM STRING>.csv
│  │  └── ACME\ Asia___<RANDOM STRING>.csv
│  └── terms/
│     ├── ACME\ Europe___<RANDOM STRING>.csv
│     └── ACME\ Americas___<RANDOM STRING>.csv
└── 73/
   ├── offers/
   │  ├── Apple___<RANDOM STRING>.csv
   │  └── Google___<RANDOM STRING>.csv
   └── misc/
      ├── Apple___<RANDOM STRING>.csv
      └── Google___<RANDOM STRING>.csv

What I had to do is port the Roda app to a Rails controller just to modify the key generation like so:

      filename = params["filename"]
      file_extension = File.extname(filename)
      filepath = filename.gsub(/#{Regexp.escape(file_extension)}\z/, "")
      key = [params["prefix"], "#{filepath}__#{SecureRandom.hex}#{file_extension}"].compact_blank.join("/")

[janko]

You should be able to override the default multipart upload key as follows:

Shrine.plugin :uppy_s3_multipart, options: {
  create_multipart_upload: -> (request) do
     filename = request.params["filename"]
     file_extension = File.extname(filename)
     filepath = filename.gsub(/#{Regexp.escape(file_extension)}\z/, "")
     key = [request.params["prefix"], "#{filepath}__#{SecureRandom.hex}#{file_extension}"].compact_blank.join("/")

    { key: key }
  end
}