I'm very used to "filename" (and that's how we tend to treat it in code - we don't inner-cap the N). For user-facing strings "file name" is probably more standard English, though for technical users it might be different - I have a suspicion that coders are probably more used to "filename", though I don't have any data.

(FWIW, I did a search in Adobe's help docs, and it seems like the documentation itself usually uses "filename", but when it refers to label strings in the UI they tend to be "File Name", probably because they need to be capitalized and "Filename" looks weird. That doesn't apply to this case anyway since it's uncapitalized in a message string, not being used as a label.)

Anyway, I guess after all that I don't have a strong opinion either way :), as long as we're consistent. I didn't see any other user-visible uses of "filename" in the current strings file, so I think this one could go either way, and we can follow it elsewhere. Personally, I think it's fine to keep it as "filename".