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Files are uploaded as file attachments to a record during an API_AddRecord or API_EditRecord call. The file field you use for file attachments must be of type File.
The file must be base64 encoded. The trick is to NOT use MIME style encoding with newline characters at a maximum line length of 76. Instead, QuickBase requires you omit these newlines. If you dont omit the newlines, the stored file in QuickBase wont be usable.