Sebastian this is file manager, included with core, we're talking about not a webpage. I upload a file and try to download same file. I know it is not corrupted during upload via file manager, only on download via file manager.
In file manager if you hover over the file you want to download, it shows:
_dopostback('dnn$ctr362$FileManager$dgFileList$ctlxxlnkDLFile',") where xx changes with file your pointing to.
I've tried fidler but don't really see abnormalities
These are standard, not secure, in file manager and it doesn't matter whether it's in root or subdirectory and I've tried 3 different browsers on 3 different machines.
The DNN 6.2.2 install on same server, with same compression settings works fine. It's just 6.2.4 that don't work. I have another 6.1.5 install on same server and it also works.
The file is about 40% bigger on download than it was on upload. The file size when looking at server is same as local computer. It is the download process in file manager that increases the size.