Fix nuxeo-document-preview performances

        static get properties() {
          return {
            /**
             * Document
             */
            document: Object,
            /**
             * By default it will display a preview of the main file.
             * For example using `xpath="files:files/0/file"` will display the preview of the document's first attachment.
             */
            xpath: {
              type: String,
              value: 'file:content',
            },
            _blob: Object,
            _imageSource: {
              type: String,
              computed: '_computeImageSource(document)',
            },
            _videoSources: {
              type: Array,
              computed: '_computeVideoSources(document)',
            },
            _storyboard: {
              type: Array,
              computed: '_computeStoryboard(document)',
            },
            _audioSource: {
              type: String,
              computed: '_computeAudioSource(document)',
            },
            _iFrameSource: {
              type: String,
              computed: '_computeIFrameSource(document)',
            },
          };
        }
        static get observers() {
          return [
            '_updateBlob(document, xpath)',
          ];
        }