Problem having my iMac recognize the video format to transfer from my iPhone 7.

Does anyone know why my iMac will not allow me to download videos taken with my iPhone 7, I already know it does not recognize the video format. I don't know if I can change that somewhere or not. It always used to transfer my videos! If anyone has information on this would help. I do have an older iMac. 2008 version with OS X El Capitan version10.11.6 I am beyond frustrated! I have no more space in my phone due to not wanting to lose my videos! I have large data storage too! I have pics of my daughter naked in a tub as an infant, and I do not want to upload my pics to the cloud due to this fear of some freak capturing her image! So if anyone can help me out I'd be greatly appreciative!