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ServiceNow Store - OpenText Documentum/D2 Connector
This partner has a potential solution
This might help also.
GitHub - aldago/DctmRestJava11
you can also create your own rest controller that receives base64 content and then stores it… If you have any minimal development capabilities, this should be an extremely simple customization. Just be aware that base64 encoding is not memory friendly and also what you want to do is not really pure "restful", but if you can live with that it should be your best option…