Hi Sergey!
Have you tried browsing to http://localhost:50002/OneStreamApp/SVC/XFFileSystem.svc you might find more info.
Also did you check that the extension is on the filename? My understanding is that the fileBytes is already good on your side? Have you tested it? for example using https://base64.guru/converter/decode/file
Or Have you tried with the exact same base64 (byte) than with the other file or see if there is a difference? Have you hard coded the fileName instead to "test.csv" or "test.xls" depending on the filetype?
'Export Register
Dim fileName As String = "Register.csv"
'Export CSV to User Temp Folder
Dim csv As String = Me.CreateRegisterTableCSVText(si, profileName, scenarioName, timeName).ToString
Dim fileBytes As Byte() = Encoding.UTF8.GetBytes(csv)
BRApi.Utilities.SaveFileBytesToUserTempFolder(si, si.UserName, fileName, fileBytes)
