I've looked at the file through a few tools, and it genuinely appears to have been partially corrupted. All the basic information--e.g. name, tileset name, layer sizes, help strings, animated tiles--is fine, and even all the events are fine, but I can't find any way to access any information about which tiles were placed where. Some of the bytes in the latter half of the file have been overwritten with other ones, and I don't know why but nor do I know any way to recover the original information.