Cause:
In our attachment control, the default size is generally 10M. If we set 30M, uploading will not be satisfied.
the reason:
It is because we take the value of the custom size of our control in a file of our ajax, which cannot be obtained. Because we passed the value of type int, the value when we fetched with QueryString is Null.
solve:
Open the Portal->Ajax->FileUploadHandler.ashx file and find the following code:
ProtectedintMaxSize
{
get
{
if(!int.TryParse(Request.QueryString["MaxSize"],Out _maxSize))
{
_maxSize = 10 * 2014;
}
return_maxSize;
}
}
We replace the above corresponding Request judgment part with the following code:
Request.Params["MaxSize"]
You can implement custom attachment upload control properties.
Effect display diagram: For 
details, please refer to this article I wrote: http://blog.ncmem.com/wordpress/2019/08/12/%e5%a6%82%e4%bd%95%e8%a7% a3%e5%86%b3%e6%8e%a7%e4%bb%b6%e9%99%84%e4%bb%b6%e4%b8%8a%e4%bc%a0%e6%97%b6% e8%b6%85%e5%a4%a7%e9%99%84%e4%bb%b6%e6%97%a0%e6%b3%95%e4%b8%8a%e4%bc%a0%e7% 9a%84%e9%97%ae%e9%a2%98/