Why doesn't Response.TransmitFile return my file in my WCF service?

Question

If you could explain me why TransmitFile ain't working, code is simple, I create excel file (.xlsx), save it on server and return file path (via CreateProjectsReport), and then I just want to take file and transmit it to user ....

string filePath = CreateProjectsReport(ProjectIds, items);

System.IO.FileInfo file = new System.IO.FileInfo(filePath);

if (file.Exists)
{
    HttpContext context = HttpContext.Current;

    try
    {
        context.Response.Clear();
        context.Response.ClearHeaders();
        context.Response.ClearContent();
        context.Response.AddHeader("Content-Disposition", 
                                   "attachment; filename=" + file.Name);
        context.Response.AddHeader("Content-Length", file.Length.ToString());

        // context.Response.ContentType = "application
        // vnd.openxmlformats-officedocument.spreadsheetml.sheet";

        context.Response.ContentType = "application/vnd.ms-excel";
        context.Response.TransmitFile(file.FullName);
        context.Response.Flush();
        // context.Response.WriteFile(file.FullName, false);                                
    }
    catch (Exception ex)
    {
        LogException(ex);
    }
    finally
    {
        System.IO.File.Delete(filePath);
        context.Response.End();
    }
}

Result of this code is that file get saved on my server in expected folder (full report is there, I verified excel file that is being generated by method) and on response.Flush() I see file being transmitted on browsers with content length of 30kb as the file length is on server, but once i click save and file gets saved in my download folder it's length is 10kb, opening it with excel I get error: Excel foudn unreadable content in 'filename.xlsx'. Do you want to recover .....

Checking IIS MIME Types, it's set (I tried using both for content type, get same error):

.xls - application/vnd.ms-excel

.xlsx - application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

Does context.Response.TransmitFile suppress my file and doesn't send full content of it? Why does this happen?

EDIT

Through browsers console I can see that in Response.Header property content-length is exact length of file, but once i click Save file, that file size goes from 1kb to 15kb tops, instead of 30kb which is on server (it's like TransmitFile(file.FullName) doesn't Transmit whole file