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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...
This is a longstanding bug, not a feature request, that DropBox has claimed is not an issue with DropBox but with iOS, and yet DropBox is the only source that has this issue on iOS -- not Google Drive, FileExplorer, iCloud Drive, etc etc. That's fine; the longer issues like this persist, with DropBox refusing to fix them or even acknowledge it's an issue of theirs, the more people will move to using other services to store and share files, out of necessity.