Проблема в том, что RestSharp автоматически превращает мои XML-файлы в Json, пытался использовать clientFile.AutomaticDecompression = false;
{"id": 5590, "idNumber": "2018_799", "tasksFiles": [{"id":17682, "idNumber": "2018_799_1", "languageCombination": {"sourceLanguage": {"id": 71, "symbol": "EN-US", "name": "English (United States)", "displayName":" English (United States) "}," targetLanguage ": {" id ": 186," symbol ":" ES-AR "," name ":" Spanish (Argentina) "," displayName ":" Spanish (Аргентина) "}}," inputWorkfiles ": {" name ":" Входные файлы "," directoryies ": []," files ": [{" name ":" Venga_Coffee_en-US.xml "," id ": 87363, «скачиваемые»: true, «категория»: «Рабочий файл», «zip»: false, «fileStats»: {«charactersWithSpaces»: 25085, «charactersWithoutSpaces»: 23944, «слов»: 1042, «строк»: 104, "pages": 0}}]}, "inputResources": {"name": "Resources", "directoryies": [{"name": "Reference File", "directoryies": [], "files":[{"name": "Rigi Magic", "id": 87362, "downloadable": false, "category": "Ссылочный файл", "zip": false, "fileStats": {"charactersWithSpaces": null, "charactersWithoutSpaces ": нулевые," слова ": нулевая," линия ": нулевая," страница ": нулевая}}]}], "files": []}, "output": {"name": "Output Files", "directoryies": [], "files": [{"name": "Venga_Coffee_en-US.xml", "id ": 87375," downloadable ": true," category ":" Work File "," zip ": false," fileStats ": {" charactersWithSpaces ": 3880," charactersWithoutSpaces ": 3052," words ": 611,"lines ": 101," pages ": 0}}]}," outputFilesAsZipDownloadable ": true}]," outputFilesAsZipDownloadable ": true}