Q: .avi and .mov file on iMac changed to .m4v when copied onto external hard drive. Will I be able to open these files on other devices like a Windows PC?

FWIW, file extensions don't change just by copying files to an external drive.

Let me put it a different way. I know of no mechanism in OSX that could do that. I've never seen it or heard of it in 30+ years.

How are you copying the files? Just dragging them over to the external drive?

Perhaps someone else will have an answer. To answer your original question, do the "changed" .m4v files play on your Mac? If so they should play on Windows. I don't what to say about a file that changed to a .jpg extension.

Where are you dragging the .avi and .mov files from?  If they are in a Photos library and you're dragging the thumbnail to the external HD the format will be changed to the .m4v format. 

To keep the original format select the video files and use the File ➙ Export ➙ Export Unmodified Original for X Videos menu option.

Interesting. I can't even add an .avi file to Photos and when I drag a .mov file from Photos to my desktop it keeps the .mov extension.

beats me.