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See https://www.dropboxforum.com/t5/Files-folders/Filenames-become-lowercase-when-attaching-from-DropBox-to-Mail/td-p/349737. Filenames should not become all lowercase when exporting a file from Drop...
Incredible that Dropbox support insists on calling this, an obvious bug, a "Feature Request". Imagine how long it'll take to get actual features in the iOS Files app.