Unable to transfer video files to external

Hi all,

I've been struggling with this for a while and can't seem to find the solution.

I imported photos and videos from my iphone into the Photos app on my macbook pro OS X Yosemite (10.10.5). I can drag and drop photos onto my external without a problem. However, videos will not transfer - there is no error message or any indication they have not been transferred. I have noticed all the videos that will not transfer are snapchat videos I have saved. Videos that others send to me are transferred without a problem.

I can't understand what the problem is. Further, if I try to copy and paste the video files into another folder on finder or onto the external they show up as jpg files despite the fact that they play completely fine in Photos. The file title is something like: thumb__recorded-1502237885_1024.jpg. So a thumbnail of the video can be copied and pasted but not the video file itself.

My external is set up for macs and i have reading and writing permissions. Please help!