Fixes #6957,BZ1121950 - Using small file upload chunks

Using smaller chunk sizes for file uploads. In production, we're getting 413
responses. It looks like the SSL negotiation (which is limited by
SSLRenegBufferSize) is hitting its limit. After some trial-and-error, the limit
seemed to be around 5-6 MB so I'm adding in some buffer and using 4 MB.