Graphql server has an already built-in Upload scalar to upload files using a multipart request.
It implements the following spec https://github.com/jaydenseric/graphql-multipart-request-spec, that defines an interoperable multipart form field structure for GraphQL requests, used by various file upload client implementations.

To use it you need to add the Upload scalar in your schema in order for gqlgen to automatically add the marshalling behaviour to Go types, and add the MultipartForm transport to the Graph server.

Configuration

Example:

c := Config{ Resolvers: &resolvers{} }
srv := handler.New(generated.NewExecutableSchema(c))

// Add the multipart form transport
srv.AddTransport(transport.MultipartForm{})

There are two options particularly relevant for uploading files:

MaxUploadSize
MaxUploadSize sets the maximum number of bytes used to parse a request body as multipart/form-data.
MaxMemory
MaxMemory defines the maximum number of bytes used to parse a request body as multipart/form-data in memory, with the remainder stored on disk in temporary files.

Examples

Single file upload

For this use case, the schema could look like this.

"The `UploadFile, // b.txt` scalar type represents a multipart file upload."
scalar Upload

"The `Query` type, represents all of the entry points into our object graph."
type Query {
    ...
}

"The `Mutation` type, represents all updates we can make to our data."
type Mutation {
    singleUpload(file: Upload!): Boolean!
}

cURL can be used the make a query as follows:

curl localhost:4000/graphql \
  -F operations='{ "query": "mutation ($file: Upload!) { singleUpload(file: $file) }", "variables": { "file": null } }' \
  -F map='{ "0": ["variables.file"] }' \
  -F 0=@a.txt

That invokes the following operation:

{
  query: `
    mutation($file: Upload!) {
      singleUpload(file: $file)
    }
  `,
  variables: {
    file: File // a.txt
  }
}

Multiple file upload

For this use case, the schema could look like this.

"The `Upload` scalar type represents a multipart file upload."
scalar Upload

"The `File` type, represents the response of uploading a file."
type File {
    id: Int!
    name: String!
    content: String!
}

"The `UploadFile` type, represents the request for uploading a file with a certain payload."
input UploadFile {
    id: Int!
    file: Upload!
}

"The `Query` type, represents all of the entry points into our object graph."
type Query {
    ...
}

"The `Mutation` type, represents all updates we can make to our data."
type Mutation {
    multipleUpload(req: [UploadFile!]!): [File!]!
}

cURL can be used the make a query as follows:

curl localhost:4000/query \
  -F operations='{ "query": "mutation($req: [UploadFile!]!) { multipleUpload(req: $req) { id, name, content } }", "variables": { "req": [ { "id": 1, "file": null }, { "id": 2, "file": null } ] } }' \
  -F map='{ "0": ["variables.req.0.file"], "1": ["variables.req.1.file"] }' \
  -F 0=@b.txt \
  -F 1=@c.txt

That invokes the following operation:

{
  query: `
    mutation($req: [UploadFile!]!)
      multipleUpload(req: $req) {
        id,
        name,
        content
      }
    }
  `,
  variables: {
    req: [
        {
            id: 1,
            File, // b.txt
        },
        {
            id: 2,
            File, // c.txt
        }
    ]
  }
}

See the _examples/fileupload package for more examples.

Usage with Apollo

apollo-upload-client needs to be installed in order for file uploading to work with Apollo:

import ApolloClient from "apollo-client";
import { createUploadLink } from "apollo-upload-client";

const client = new ApolloClient({
	cache: new InMemoryCache(),
	link: createUploadLink({ uri: "/graphql" }),
});

A File object can then be passed into your mutation as a variable:

{
  query: `
    mutation($file: Upload!) {
      singleUpload(file: $file) {
        id
      }
    }
  `,
  variables: {
    file: new File(...)
  }
}