Simplest method to transmit a file to FPGA

PCIe would be the obvious choice, but that's complicated and potentially expensive if you're not using a device that has the hard PCIe IP built in. For simple and cheap you can't beat a UART. In this case you can't read the file out of PC memory, you would need an application on the PC transferring the file to you using the UART. But you can use terminal emulators like Teraterm to do this. UARTs are slow, but pretty darn easy to implement and to debug. You can at least star there to get the ball rolling and then upgrade to improve performance as needed.