can malware spread when sending a clean file to someone or receiving it via bluetooth ?
As you mentioned, malware uses pair-wise communication mechanisms such as Bluetooth to spread. But whether the file you want to send is clean or not does not prevent you (or the destination deice) from being infected because one of the common ways that malware uses to propagate is to exploit vulnerabilities in the Bluetooth communication stack as you can read further in A Preliminary Investigation of Worm Infections in a Bluetooth Environment
If so, does it need some action from the user to start the malicious code ?
There is malware that depend upon user interaction for a successful infection. We can mention, for instance, the famous Cabir worm that propagates over Bluetooth using caribe.sis that displays a warning message:
This worm requires the user to click on Yes to get installed successfully. But as I said before, not all worms work need user interaction like this.
