feat: find image-only documents by their filename

A photograph has no text to read, so an inspection photo named "Shattered
roof skylight leaving the interior exposed to weather elements" could not
be found by a search of the matter - recognition cannot help a picture
with no words in it. Now, when a document yields no text of its own, its
filename is stored as a single searchable passage, so the photo is found
by that description, by exact words or by meaning like anything else. A
result says plainly when the match was on the file's name rather than on
text inside the file, so the assistant does not quote a picture as if it
held words.