tmp file extension (archive_name.tmp)

TMP file extension stands for TEMP, so it's not really a file extension per se, it's more like a temporal file with some content inside. They are generally temporal so you don't have to mess with them.
In case they are not temporal (you reboot you machine and the file is still there) you can delete it as it was a left over (the application that created it forgot to erase it).
If you are curious and want to see what is inside you first have to know what kind of file it's, for example it's a text document? an image?. If you want a sneak peek you should use the Notepad or Notepad ++ it will give you an idea of what is the file content