Fix "Key not found" exception when name and filename parameters are omitted

[nakamura2000]

if parameters have only content-type and don't have name parameter and filename parameter, ParseParameterPart throws key not found Exception. ( parameters["name"] ). Use ParseFilePart in this case.

[Jericho]

At first I was skeptical about your suggestion to treat a parameter without name as a file. I was concerned that it could potentially change how we treat some parameters but I realized that the name is indeed required to uniquely identify "data" parameters and therefore a parameter without name cannot be a "data" parameter. Currently, as you reported, we throw an exception when a parameter is missing the name which means that the parsing of the form data completely fails.

Your PR allows the parsing to be successful even if a parameter does not have a name in addition to supporting the mjpeg stream scenario you mentioned in the issue you raised.

Overall, I think this is a great improvement so I will merge your PR, add a few unit tests and release an update shortly.

Thank you for your contribution!