The maximum Filesize (Request-Length) is not defined by DMX but by DNN in web.config.
Look for followed line in your web.config:
<
httpRuntime useFullyQualifiedRedirectUrl="true" maxRequestLength="8192" requestLengthDiskThreshold="8192" />
The maximum Request-Length is set per default in web.config to 8192 = so 8mb you must set the maxRequestLength and requestLengthDiskTreshold to a higher Value.
But you cannot calculate so that you set 8mb an can upload 8mb because when you have a slow upload you must set the value higher.
This falues are important to prevent attacks of your Portal!
Please consider when you made a update to a new version from dnn you must every check this line and set to the value what you in the version before!
I hope this helps.