fix: correct regex for binary types

[Xenonym]

Summary

The regex to match binary MIME types had a typo (applicaton/octet-stream, note the missing i). This PR fixes that by correcting the regex so that application is only spelled once.

This PR also follows the behavior documented in this forum answer where all application/* content types are base64 encoded except for some known text types:

The fix

Going forward, we will base64 encode the body if the content-type starts with application/. This will ensure that binary media types like application/octet-stream and application/pdf arrive at their final destinations intact.

Exceptions

Requests with the following content-types will not be base64 encoded, even though they start with application/:

• application/json or application/*+json
• application/xml or application/*+xml
• application/javascript
• application/csp-report
• application/graphql
• application/x-www-form-urlencoded
• application/x-ndjson

A picture of a cute animal (not mandatory, but encouraged)
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[Xenonym]

Closing in favour of #3631.