Tags with NULL values - how to substitute another tag?

[CRH]

I'm processing cards which contain both images and videos, embedding the date of capture into the filename.

{@prefix,1,-1,_}{exif.DateTimeOriginal}_{@filename}

This works for the images, but the movies to not have EXIF data, so it shows up blank, and I get "CRH__P1234567.MOV".  I'm using the EXIF date because, if I pre-process the files with another tool prior to using ImageIngester, the file date and time are no longer the same as the capture date and time.

What I'd like to be able to do is use exif.DateTimeOriginal, then fallback or coalesce to a different tag, like the file timestamp.

I'm thinking something along the lines of:
{@prefix,1,-1,_}{exif.DateTimeOriginal||@datetime}_{@filename}

Is this supported?

Thanks,
clay.