Deserialization fails with multipart/form-data

[hsundell]

Made a PR that replicates the issue here with more discussion: #198

I'm not too familiar with the inner workings of the module but now that it's possible to replicate the issue, theoretically I could also try to resolve it. But maybe someone more knowledgeable in the module would fix it much faster/better.

[Jmennius]

Might be fixed by python-openapi/openapi-core#646 (not released at the moment)

[zupo]

This should be fixed by #225, included in release 0.17.1.

Let me know if the issue persists.