How can I restore a video to my camera roll in a way iMovie can find it again?

Using iPhone 6+, I recorded a video that saved to my camera roll as usual. I used that video to create a movie in iMovie on that same phone. Then I imported all the phone's videos into Photos on my MacBook Pro and, not thinking, told it to delete imported files. When I went back to iMovie on my phone, it told me the original video was missing and I had a lot of yellow triangles on ever clip.

I tried:

1. Exporting the video from Photos and uploading to Dropbox so I could download it back into my camera roll. iMovie still said it couldn't find it.

2. Syncing the video to the phone using iTunes. However, I cannot get the file back in it's original location (Camera Roll) using this method.

From attempt #1 listed above, the original video is back in the camera roll and it's still not finding it which makes me wonder if the file name is somehow different? I don't know what an original video file name looks like when it's first created in the camera roll? Does it have an extension like .mov or not? Etc?

Any ideas?