You can use the ftp server to upload big files. Upload them to
ftp://ftp.avast.com/incoming - Using Internet Explorer, Connect to the link and drag the file into the Right pane and drop it, that starts the upload, you don't have read access to this folder.
How to upload it using the Run command-line in Windows: Windows Key + R (to get the run box), copy and paste this
explorer ftp://ftp.avast.com/incoming
and drag the file into the window, from another explorer window.
Give the file a meaningful name UndetectedMalwareSamples, etc.