Your camera records AVCHD files. Don't rename the file extensions.
What version of After Effects are you using? Can you open the files in Adobe Premiere Pro? Did your camera come with any file conversion utilities?
These files have been notoriously problematic. I've seen a couple of posts on this over the years. Though this may still be a version specific issue, but since you didn't tell us any details, we can't advise. Anyway, as Andrew said, don't make matters worse by renaming them to arbitrary extensions. Instead verify the files in other tools and try to transcode them in other tools like ffmpeg/ MPEGStreamclip/ Handbrake, if nothing else works.