In the WebForms inspector the "Content-Type: multipart/form-data is not yet fully supported." label means that it is possible that the WebForms inspector could not show all of the data in the request body.
However, you could still use other inspectors like the Raw or the TextView, to be sure that you inspect all the data of the request body.
If you prefer to inspect the multipart/form-data in the WebForms inspector, please, make a request at https://fiddler.ideas.aha.io/
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