strapi/packages/strapi-plugin-upload/config/schema.graphql

const _ = require('lodash');

module.exports = {
  mutation: `
    upload(refId: ID, ref: String, source: String, file: Upload!): UploadFile!
  `,
  resolver: {
    Query: {
      file: false,
      files: {
        resolver: 'Upload.find'
      }
    },
    Mutation: {
      createFile: false,
      updateFile: false,
      deleteFile: false,
      upload: {
        description: 'Upload one or many files',
        resolver: async (obj, { file, ...fields}, { context }) => {
          // Construct context to fit with koa-parser guidelines
          // and avoid to update our business logic too much.
          context.request.body = {
            files: {
              files: await file
            },
            fields
          };

          // Call controller action.
          await strapi.plugins.upload.controllers.upload.upload(context);

          // Handle case when the user is uploading only one file.
          if (_.isArray(context.body) && context.body.length === 1) {
            return context.body[0];
          }

          // Return response.
          return context.body;
        }
      }
    }
  }
};