strapi/packages/strapi-plugin-content-manager/config/policies/routing.js

const _ = require('lodash');

module.exports = async (ctx, next) => {
  const { source } = ctx.request.query;

  if (source && _.get(strapi.plugins, [source, 'config', 'layout', ctx.params.model, 'actions', ctx.request.route.action])) {
    const [ controller, action ] = _.get(strapi.plugins, [source, 'config', 'layout', ctx.params.model, 'actions', ctx.request.route.action], []).split('.');

    if (controller && action) {
      // Redirect to specific controller.
      if (ctx.request.body.hasOwnProperty('fields') && ctx.request.body.hasOwnProperty('files')) {
        let {files, fields} = ctx.request.body;

        const parser = (value) => {
          try {
            value = JSON.parse(value);
          } catch (e) {
            // Silent.
          }

          return _.isArray(value) ? value.map(obj => parser(obj)) : value;
        };

        fields = Object.keys(fields).reduce((acc, current) => {
          acc[current] = parser(fields[current]);

          return acc;
        }, {});

        ctx.request.body = fields;

        await strapi.plugins[source].controllers[controller.toLowerCase()][action](ctx);
        const resBody = ctx.body;

        await Promise.all(Object.keys(files).map(async field => {
          ctx.request.body = {
            files: {
              files: files[field]
            },
            fields: {
              refId: resBody.id || resBody._id,
              ref: ctx.params.model,
              source,
              field
            }
          };

          return strapi.plugins.upload.controllers.upload.upload(ctx);
        }));

        return ctx.send(resBody);
      }

      return await strapi.plugins[source].controllers[controller.toLowerCase()][action](ctx);
    }
  }

  await next();
};