Cloud backup is - quite obviously - a specific solution that needs to upload files somewhere... otherwise it wouldn't be much of a backup.
The original question was about an antivirus (scanning). Uploading the content of every scanned file (anywhere) would be terribly slow and expensive - so that certainly doesn't happen. The following can happen:
1. To query a file reputation or classification, Avast may compute some kind of short description of the scanned file (such as a hash, information about the digital signature etc.) and ask Avast servers to see what we know about that file. It's certainly not the full content, and it certainly doesn't happen e.g. for every file during the full disk scan - again, that would be very slow.
2. If the scanner heuristics decides that a scanned file is somehow suspicious but it's not clear whether it's malicious or not, the file content can be uploaded for additional Virus Lab investigation - either as part of the CyberCapture feature, on program execution (see the Core Shields section in settings), or as a "suspicious file submit" during various scans if the "Community IQ" is enabled (see the first option on Personal Privacy page in settings). It's also not a common issue and the file has to be somehow rare/strange.
