File name as primary key?
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.