First, Every upload form has a Max file size value, Like the Image below,
Its in kilobyte. So Please make sure you put your desired value here. For 100 Megabyte, The Value will be 102400. If its less then 50 Megabyte, That will show a Error. But that shouldn't be a Error # -200 HTTP Error .
Another Reason of this problem could be, if you are using HTTP authentication for the site. Like Password Protected Direcotry. It sometime cases the problem.
Or, Your php upload limit is less then 50 Megabyte. WPUF can't override the default upload limit in php. You should be able to find it in your php.ini. In case if you are using a shared hosting, You might not have a change to edit or Override the Default settings of php.ini.
Please inform us after you check those scenario i mentioned above.