clearRelatedCache causes crash when model contains file collection

[josias-milk]

When clearRelatedCache is enabled and a model contains a field which is a file collection, the following crash occurs:

TypeError: Cannot read property 'kind' of undefined
      at /Users/foo/myproject/node_modules/strapi-middleware-cache/lib/index.js:195:51
      at /Users/foo/myproject/node_modules/lodash/lodash.js:4967:15
      at baseForOwn (/Users/foo/myproject/node_modules/lodash/lodash.js:3032:24)
      at /Users/foo/myproject/node_modules/lodash/lodash.js:4936:18
      at Function.forEach (/Users/foo/myproject/node_modules/lodash/lodash.js:9410:14)
      at clearCache (/Users/foo/myproject/node_modules/strapi-middleware-cache/lib/index.js:194:13)
      at async /Users/foo/myproject/node_modules/strapi-middleware-cache/lib/index.js:248:9
      at async bustAdmin (/Users/foo/myproject/node_modules/strapi-middleware-cache/lib/index.js:318:11)
      at async /Users/foo/myproject/node_modules/strapi/lib/middlewares/responses/index.js:9:9
      at async cors (/Users/foo/myproject/node_modules/@koa/cors/index.js:95:16)
      at async /Users/foo/myproject/node_modules/strapi/lib/middlewares/logger/index.js:56:11
      at async /Users/foo/myproject/node_modules/strapi/lib/middlewares/responseTime/index.js:17:9
      at async /Users/foo/myproject/node_modules/strapi/lib/services/metrics/middleware.js:29:5
      at async /Users/foo/myproject/node_modules/strapi/lib/Strapi.js:325:9

In my project I have a field called imageGallery which looks like this:

    "imageGallery": {
      "collection": "file",
      "via": "related",
      "allowedTypes": [
        "images"
      ],
      "plugin": "upload",
      "required": false,
      "pluginOptions": {}
    }

I think the problem is in the getRelatedModels function, line 102ff:

    if (attr.collection) {
      relatedModels[attr.collection] = models[attr.collection];
    } else if (attr.model && attr.model !== 'file') {
      relatedModels[attr.model] = models[attr.model];
    }

In the first case, when attr.collection is true, I think it should also check that the collection != 'file'