The audit log confirms the route through the code as expected - unfortunately that doesn't give me any extra information on the issue.
The same code paths are used for uploading other files.
saveFile for "Open Sans Regular" with storedAs = "", fileName = "OpenSans-Regular.ttf" to "79.ttf". Always Copy = "", Cloned = ""
Can you confirm that the file left in
temp/ is named "OpenSans-Regular.ttf" ?