Attachment attempts (three variations):

ZohoBooks_add_expense_attachment

path_variables: { "expense_id": "3971503000000806001" }
query_params: { "organization_id": "806493308" }

// Attempt 1 — original file, backslash path (74 chars)
body: { "attachment": "C:\\Users\\quint\\OneDrive\\HITS\\hits-reciepts\\Scan - 2026-01-07 10_16_43.pdf" }
→ Error 33003: "Receipt not attached."

// Attempt 2 — archived file, forward slashes (103 chars)
body: { "attachment": "C:/Users/quint/OneDrive/HITS/hits-reciepts/archive/2026/2026-01-07-10-12-petersons-north-branch-mill.pdf" }
→ Error 15: "Please ensure that the name of the attachment has less than 100 characters."

// Attempt 3 — shorter path, backslashes (85 chars), with totalFiles
body: { "attachment": "C:\\Users\\quint\\OneDrive\\Documents\\Claude\\Projects\\Bookkeeper\\2026-01-07-petersons.pdf", "totalFiles": 1 }
→ Error 33003: "Receipt not attached."

The pattern on the attachment: error 15 fires when the path string is over 100 chars, error 33003 fires when it's under. That suggests the MCP is validating/passing the path length but not actually reading and sending the file bytes in the multipart body.