Adding an attachment to an individual feature that is over 500 MB results in the following error (the table name is variable):
Data loaded into attachments must fit into memory. ArcGIS Desktop is a 32-bit application, so it is limited to 2 GB. The memory required for loading must be continuous. So the size limit for an attachment is dependent on how much continuous memory is available. With most configurations this results in a limit around 500 MB per individual file.
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