That's a tape archive!
Specifically ustar, the modern kind. See
tar (file format)
Each file record begins with a header containing, among other things, a filename and 'ustar'. The literal contents of the file follow immediately after, padded to a certain block size.
This is good news. If you can figure out where the file begins and ends, you can dump it and extract it with ordinary tools, and build a new one from scratch. (It'd be best to do so on a UNIX system though, because it contains important file permissions, something a Windows system would botch mercilessly.)
You might also be able to append things without altering the previous content; tar, being originally built for tapes, plays well with that.