MCP Can't Upload Files — Here's What I Learned Building a Production Workaround

TL;DR Connected MCP to a cloud accounting API → expense registration went from 8 hours to 30 minutes Tried to upload a receipt → 400 error (MCP can't do multipart/form-data ) This is a protocol-level limitation , not a server bug Built CLI Skills (curl + jq) as a workaround → 90% token reduction + full file upload support The Setup I'm a freelance engineer. Every year, expense registration in cloud accounting software takes hours of manual work — checking receipts one by one, entering amounts, selecting account categories, uploading images. Last year it took about 8 hours. This year, I connected MCP (Model Context Protocol) to the accounting API via Claude Code. The AI reads receipts, extracts amounts, determines account categories, and registers everything automatically. 8 hours → 30 minutes. I was thrilled. Then I tried to attach receipt images. The 400 Error Tool: mcp_server__api_post Parameters: path: /api/v1/receipts body: {"company_id": "xxx", "description": "Electricity July"} R