Better default value for Binary Mode

[vladiulianbogdan]

I have a question regarding the Binary Mode. Could we set some better default values for binary? For example, application/pdf will just not work by default. The user has to set by itself the BINARY_CONTENT_TYPES env variable.

If you agree that we could set some better defaults, I can come with a PR for this.

[dougmoscrop]

Yes that makes sense to me. What do you think about going the inverse and having a list of known text types?