Intecreate upload should reject files that don't end in .bin

[JPHutchins]

We can't help if someone renames a .hex to .bin, but we can convert hex -> bin and/or simply reject anything that's not bin. The regular upload and upgrade commands use the MCUBoot tool from smpclient to inspect for image validity and convert from hex -> bin. Here we don't care if it's an MCUBoot valid image but it could be converted to bin similarly.