Draft: probe content type of files using different strategy (i.e. inspect first bytes)

[HoneyryderChuck]

current implementations are relying on file name/extension, which is prone to errors (.mp4, is it a video or a picture?) and spoofing. This change allows customers to use or opt into mechanisms which inspect the first bytes of a file's payload.

I'd like to do the same change for other http clients we use, but I'd like to feedback to validate the viability of this approach first.

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[HoneyryderChuck]

cc @martin-mfg @lwlee2608

[wing328]

thanks for the PR

can you please add some tests in https://github.com/OpenAPITools/openapi-generator/blob/5a18e9897bd8b2b4a5013a41bccaf93710dd3883/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java to test guessContentTypeFromFile with different file types to confirm the enhancement is good?

[HoneyryderChuck]

I've added them, but it seems that the default implementation for FileTypeDetector relies on file extension (commented test).

I'm not very knowledgeable on java and the description on how to change the implementation, so I'd need some help from someone who is in order to change the implementation to accomodate the commented test, but I'd say that, as long as we use this API, users can at least do it.

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/9585429177/job/26431446540?pr=18949

please update the samples when you've time

[wing328]

shall we use URLConnection.guessContentTypeFromStream instead as suggested in https://stackoverflow.com/a/19712111/677735?