File name as primary key? I have a few tables I'm going to create for a website, Literature, video, and image. Is there a good reason why I shouldn't use the file name as a primary key.

For instance...

"someimage.png" as the PK for table image.

The only downside I can think of is if I had named someimage.png in multiple directories and each directory someimage.png was not the same image.  This seems like poor design and there isn't any reason why I'd do this anyways. 

Thoughts?
In general, surrogate PKs are preferable compared to natural. Usually, primary key columns never get updated; in your case renaming file will require updating primary key which in turn may cause cascade updates. Aside from that, having a long primary key is never a good idea from storage and performance point of view. 

Depends on RDMS you use, primary key may also define how the rows stored (the physical order), so even inserting will cause many more page splits and high level of fragmentation.  

Summary .I'd strongly recommend not to use file names as primary keys. If they have to be unique, you always have an option to create unique constraint.