I realize it's been five years since the last message here, but wanted to chime in with a workaround for a particular use case.
Ideally there would be a way (most likely kernel param) for the installer to either ignore DHCP filenames entirely or to carry on without complaint if it encountered one that it considered malformed. This would allow users who don't have control over the DHCP server to carry on.
I ran into this behavior with a libvirt/chainloaded ipxe setup. I needed the libvirt provisioned dnsmasq instance to set the DHCP filename to power the ipxe chainloading, but this was breaking Debian installs due to the installer trying to interpret the ipxe chainloading file as a preseed file. I was able to work around it by *not* setting the bootp element in the libvirt network xml and instead setting the following dnsmasq:options:
<dnsmasq:options>
<dnsmasq:option value="dhcp-vendorclass=debianinstaller,d-i"/>
<dnsmasq:option value="dhcp-boot=tag:!debianinstaller,
http://192.168.234.1/chain.ipxe"/>
</dnsmasq:options>
Note that the dnsmasq:option was added to libvirt in 5.6, so you need a fairly new version at the time of writing.
This will set the DHCP filename for any client *not* identifying itself with the "d-i" vendor class, as explained in the Debian B.2. Using preseeding appendix.
Sorry, this is specific to a particular use case and somewhat tangential to the original report but the problem is obscure and this might help someone.