Cannot upload attachment,"java.lang.NullPointerException"

Hi everyone,

I’m new here though I know this could be an old question…

the problem is when I try to add an attachment I get this error:
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My cmdbuild version is 3.1
Alfresco version is 6.2.0-ga (which is deployed by docker compose),
They’re running on the same server, so I changed the alfresco proxy port from 8080 to 8888, and its share from 8080 to 8081, they are all working well.

I tried to use alfresco api to upload a file to folder “cmdbuild” which I created to test, and it succeeded.
code: (you can see “isFile” is “true”)
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alfresco screenshot (you can see “myjson” file which I just uploaded with api)
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my DMS setting:
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I really don’t know how to solve this problem, no idea, so I came here to ask for help, thanks!