PSA: @signalapp has been modified sometime in the past few months (I think) to send filenames of attached images.
It's not obvious it's sending filenames because they're only visible if the recipient saves the image on desktop. This can reveal the order in which photos were taken, how many photos were taken in between, if a photo was taken on a different phone, etc. It's a surprising choice for a privacy-focused app, and partially negates benefits of stripping EXIF metadata.