下载附件后，浏览器直接打开附件

public ActionResult Download(string dirRelativePath, string fileName)
{
string token = Request.QueryString["token"];
if (htTokens != null && !string.IsNullOrEmpty(token) && htTokens.Contains(Guid.Parse(token)))
{
string uploadPath = System.Configuration.ConfigurationManager.AppSettings["BPMAttachments"];
string dirAbsolutePath = uploadPath + dirRelativePath;

if (!System.IO.File.Exists(dirAbsolutePath))
{
return Content("提示：文件在磁盘上不存在");
}
htTokens.Remove(token);
//HttpContext.Response.AddHeader("content-disposition", "attachment;filename=" + fileName);
//return File(dirAbsolutePath, "application/octet-stream");
var contentType = MimeMapping.GetMimeMapping(fileName);
HttpContext.Response.AddHeader("content-disposition", "inline;filename=" + fileName);
return File(dirAbsolutePath, contentType);
}
else
{
return Content("提示：没有权限");
}
}

注意：

1.return File(dirAbsolutePath, contentType); 中contentType不能是"application/octet-stream"，需要获取文件mimetype后，指定contentType

2.浏览器支持打开的文件格式有限，例如：txt、html、png、gif。。。。。。