Honor filename capitalization when exporting

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 DropBox (e.g. to Mail via Files, or directly to Google Drive). Other sources, such as Google Drive and FileExplorer, do not have this issue.