[Bug] GraphQL multipart request - incorrect implementation

[schroda]

Specification: https://github.com/jaydenseric/graphql-multipart-request-spec

Problem

1. Expecting specific map keys
   Currently the server expects the map keys to start with 0.
   According to the specification, the client can set the keys to be anything and the server should be able to handle it.
   Due to this, libraries like https://github.com/jaydenseric/apollo-upload-client, are currently unusable.
   E.g. the mentioned library starts its keys with 1, thus, sending requests that the server can't handle.

2. Form parameter "operation"
   The server checks for the form parameter operation, while it is operations in the specification

Solution
Update current implementation to follow the specification.