It could be .iso.001, .iso.002 … or .part1.rar, .part2.rar … or .r01, .r02 … etc etc.
There’re two possibilities to extract these files. 1. These are slpitted files during compression using winrar. Put all files in one folder. Just extract the first file, it will extract everything. 2. These are splitted files using splitting software such as cutncopy, hjsplit etc. We need use them to joint all files together.
There’re some explaination links: