You cannot upload files larger than 50MB when using MyFiles.
There is no server-side fix for this, as this is a client setting. The fix for this is detailed in the following Microsoft KB: https://support.microsoft.com/en-us/kb/2668751
To increase the file size for uploads, modify the following registry key as follows:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WebClient\Parameters\FileSizeLimitInBytes
Default value: 50000000
Change value to: 500000000