As per my experience, the issue you faced could be the content-type(multipart/form-data;boundary...) is not officially supported by content-viewer page's engine.
It does support Content-Type (such as application/json, application/xml, text/plain etc.) in original/formatted mode. Other types can be viewed via hex mode.
Directly supported Type under original mode:
1. application/json
2. application/xml
3. text/plain
Note: you may also append the charset encoding to the specified content-type.
e.g. application/json;charset=utf-8
WORKAROUND
1. You can try to put an UpdateAttribute Processor
(Configure it with an attribute "mime.type" along with the supported Content-Type listed above, specifying this attribute would not change the actual content, it just tells the content-viewer to depend on such way to parse and display the flowfile's content. )
2. View the flowfile's content via Hex mode.
Hope this helps, and feel free to contact me if you have any questions.
Thanks