The Automation Send Web Request should consider the special characters while sending the request. Specifically, for a multi-part request, similar to Postman/other tool that will compute the content length correctly, etc where as Automation Send Web Request action was never designed.

With correct "boundary" payload, the request will fail with 400 Bad request error.

e.g
Because of this limitation, it is not possible to send multiform request for Jira attachments:

• https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-attachments/#api-rest-api-3-issue-issueidorkey-attachments-post