I'm sure you're searching for a developer with experience in Dart to help you upload a document file to s3 using HTTP Post and STS Token. If you read through my profile, you'll see that I have the necessary AWS credentials and permissions to access and upload files to s3. Additionally, my team and I have extensive experience working with HTTP Post requests and STS Tokens.
We at MD TechnoSoft specialize in web and mobile application development, specifically MVC for last 8 years. We have also worked on iOS and Android app development in the past 12 months. We provide best customer service and support along with long term relationship with established clients and company.
With our expertise in Dart programming language, we are confident that we can achieve successful file uploads using your project requirements. Please let us know if you would like us to discuss further or answer any other questions you may have regarding this project!