Microsoft Flow：SharePoint触发器中‘File Content’属性解码问题

I’ve spent a lot of time building custom connectors for Microsoft Flow and troubleshooting SharePoint trigger behavior, so let me clarify exactly how the File Content property is encoded and how you can fix your decoding issues.

Core Encoding Detail #

First off: the File Content value from this SharePoint trigger is Base64-encoded binary data. Even though your incoming request has a Content-Type: application/json; charset=utf-8 header, the actual file content is stored as a Base64 string inside the JSON payload—this is the standard way to pass binary data via JSON, since JSON doesn’t natively support raw binary values.

Common Decoding Roadblocks & Fixes #

If you’re struggling to decode the string, these are the most likely culprits and solutions:

• Unprocessed escape characters: Sometimes JSON parsers add escape characters like \ to the Base64 string (e.g., SGVsbG8gd29ybGQ= becomes SGVsbG8gd29ybGQ\=). Make sure you fully parse the JSON payload first to get the raw, unescaped Base64 string.
• Wrong decoding method: Use a decoder designed for Base64-to-binary conversion, not just plain text decoding:
  • In C#, use Convert.FromBase64String(fileContentString) to get a byte array, then write it to a file or process it based on the file type.
  • In JavaScript, use atob(fileContentString) for text files, or convert the result to a Uint8Array for binary files like images or PDFs.
• Missing file type context: The Base64 string represents the full binary of the file, so you need the original file’s MIME type to handle it correctly. Grab the File Content Type property from the same trigger output—it tells you if you’re dealing with a .txt, .docx, image, etc.

Quick Decoding Workflow for Your Connector #

1. Receive the JSON payload from Flow and parse it to extract the File Content string.
2. Ensure any escape characters are stripped (most JSON libraries handle this automatically during parsing).
3. Run the raw Base64 string through a Base64-to-binary decoder.
4. Use the File Content Type value to determine how to process the binary data (e.g., save as a PDF if the type is application/pdf).

Pro Tip: For testing, you can copy the File Content string into a local Base64 decoder tool (avoid using sensitive files!) to confirm it decodes correctly before implementing the logic in your connector.

内容的提问来源于stack exchange，提问作者SPO Geek