Just tested it by creating a note on mobile, adding a picture, changing the name in the square brackets and syncing. I then opened the note on the desktop version and did a save as. Got prompted to save using the original name.

So it seems that Joplin mobile is using the attach method and saves the original file name in the database as it is when when you attach the file. You can write whatever you like in the square brackets afterwards (or make it blank). The info in the square brackets is a descriptor for the link and nothing to do with the stored attachment itself, just like with URLs. Which actually answers your original question!

Any subsequent "save as" from that note on a desktop client will suggest the original filename.

Taking a photo directly in Joplin however adds the image with an empty set of square brackets (as there is no filename to use as a descriptor) and no filename is stored against the attachment. You can type what you like in the square brackets as a descriptor but when you try a "save as" on that, (on a desktop client) the filename field is therefore empty.

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