There are many, many posts of this nature in these forums. See, for example:
iphoto won’t import mov video
For many, rebuilding the XPC helper cache solves the problem off the bat. In terminal try this, and reboot:
sudo /usr/libexec/xpchelper --rebuild-cache
The above posted by Andy Ball on a thread related to a similar problem with Aperture:
Re: Import video to Aperture library on Mountain Lion
Other things to try:
1. create a fresh iPhoto library and try that. (hold down opt while launching iPhoto)
2. create a new OS X user (in system preferences) and try there.
In my case – and many cases – neither of those things work either, which means the problem is system wide. Again suggesting that the xpc helper solution might work. But for me it did not either.
There are other suggestions that there are conflicts with Perian (on Yosemite) and flip4mac. In my case uninstalling both of those didn't solve it either.
So maybe one of those solutions will work for you, results apparently vary. But this is a huge unresolved issue many others are having as well. I have a case with a senior apple advisor open, still unresolved. His suggestion of course, at this point, is an archive and re-install. I'm not ready to go there yet.
BTW, another thing he had me try was to import the file with Image Capture (which works). Then open it in QuickTime Player X. Then try exporting to 480p. Does QuickTime crash? It does for me.
My summary:
1. Files successfully import with Image Player.
2. Files play with QuickTime Player X.
3. QuickTime Player X crashes when exporting to 480p.
4. Files import with iMove.
5. Files fail to import with iPhoto (unrecognized format as with OP)
6. iPhoto import failure/errors are system wide and not specific to a library or a user.
7. rebuilding xpchelper cache and rebooting in my case does not resolve issue.
8. same videos import fine in iPhoto on a totally different system (laptop running same versions of OS X 10.10 and iPhoto 9.5.1)