Using setContentStream change the mimetype in Alfresco

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After using the setContentStream API to update content in Alfresco, it will change the mimetype to "Binary" or "Unknown" from what is was originally.

I've tried using both Buffer or UTF-8 string as the content parameter type and it didn't make any difference. Also it happens on some mimetype but not others (for example Plain Text works fine, but not XML)