Did you compact your file before exporting?
How many people are you exporting?
How big is the media file?
1) Compact the file
2) Re-export the dataset to an .ftmb files
3) Exit FTM
4) Reboot your machine.
5) Restart FTM. If it opens some file, go to File > Close and close it so that FTM has nothing else running
6) Retry File > Restore
If that doesn't work, try a test of one person with one media item to see if will do an very small file.
I just went through the procedure with a test of 22 people and two images and it worked perfectly. But I know that FTM has various problems with big files.