Invalid mime-type set using .NET client

        /// <summary>
        ///     Upload a file with API and create a document using API.
        /// </summary>
        /// <param name="fileName">Full path file to upload.</param>
        /// <param name="folder">Destination folder.</param>
        /// <remarks>
        ///     This method create the correct document "type" and upload the document,
        ///     but is not possible to see the preview.
        /// </remarks>
        /// <see href="http://doc.nuxeo.com/nxdoc/net-client/" />
        public async Task CreateDocumentWithApi(string fileName, string folder)
        {
            try
            {
                // Upload the file.
                var uploader = new Uploader(_client);
                uploader.AddFile(fileName);
                await uploader.UploadFiles();

                var document = (Document) await _client.Operation("Document.Create")
                    .SetInput(folder)
                    .SetParameter("entity-type", "document")
                    .SetParameter("type", "vmr_document_type")
                    .SetParameter("name", "Cappe03")
                    .SetParameter("properties", new ParamProperties
                    {
                        {"dc:title", "A doc"},
                        {"dc:description", "descrizione"},
                        {"vmr_document_type:cod_fornitore", "test cod_fornitore"},
                        {
                            "file:content", new JObject(
                                new JProperty("upload-batch", uploader.Batch.BatchId),
                                new JProperty("upload-fileId", "0")
                            )
                        }
                    })
                    .Execute();
            }
            catch (Exception e)
            {
                Debug.WriteLine(e);
                throw;
            }
        }

2019-02-21T11:35:33,391 WARN  [Nuxeo-Work-default-6:1351945841105414.390932306] [org.nuxeo.ecm.platform.thumbnail.factories.ThumbnailDocumentFactory] Cannot compute document thumbnail
org.nuxeo.ecm.core.convert.api.ConversionException: application/text%2Fplain mime type not supported by anyToThumbnail converter
        at org.nuxeo.ecm.core.convert.service.ConversionServiceImpl.convert(ConversionServiceImpl.java:323) ~[nuxeo-core-convert-10.10.jar:?]
        at org.nuxeo.ecm.platform.thumbnail.factories.ThumbnailDocumentFactory.computeThumbnail(ThumbnailDocumentFactory.java:92) [nuxeo-thumbnail-10.10.jar:?]
        at org.nuxeo.ecm.core.api.thumbnail.ThumbnailServiceImpl.computeThumbnail(ThumbnailServiceImpl.java:99) [nuxeo-core-api-10.10.jar:?]
        at org.nuxeo.ecm.core.api.thumbnail.ThumbnailAdapter.computeThumbnail(ThumbnailAdapter.java:58) [nuxeo-core-api-10.10.jar:?]
        at org.nuxeo.ecm.platform.thumbnail.listener.UpdateThumbnailListener.processDoc(UpdateThumbnailListener.java:67) [nuxeo-thumbnail-10.10.jar:?]
        at org.nuxeo.ecm.platform.thumbnail.listener.UpdateThumbnailListener.handleEvent(UpdateThumbnailListener.java:137) [nuxeo-thumbnail-10.10.jar:?]
        at org.nuxeo.ecm.core.event.impl.AsyncEventExecutor$ListenerWork.work(AsyncEventExecutor.java:221) [nuxeo-core-event-10.10.jar:?]
        at org.nuxeo.ecm.core.work.AbstractWork.runWorkWithTransaction(AbstractWork.java:437) [nuxeo-core-event-10.10.jar:?]
        at org.nuxeo.ecm.core.work.AbstractWork.run(AbstractWork.java:357) [nuxeo-core-event-10.10.jar:?]
        at org.nuxeo.ecm.core.work.WorkHolder.run(WorkHolder.java:57) [nuxeo-core-event-10.10.jar:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_191]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_191]
        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]