How to use HTTP action in Power Automate with multipart/form-data (file upload + schema parameter)?

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Posted on by CU26061342-0

Hi Team,

I’m trying to implement an HTTP action in Power Automate to send a request with multipart/form-data, where I need to:

Upload a file attachment

Pass additional parameters (e.g., schema name or metadata fields)

Requirement:

I need to call an API endpoint that expects:

Content-Type: multipart/form-data

One part: file (binary content)

Other parts: form fields (like schemaName, etc.)

What I have tried:

I attempted using the HTTP action with custom headers and body, but I’m facing issues with:

Correct formatting of multipart boundary

Passing file content correctly from Power Automate trigger/output

Combining file and form parameters in the same request

Example requirement:

Something like:

--PowerAutomateBoundary
Content-Disposition: form-data; name="schemaName" 

schema1
--PowerAutomateBoundary
Content-Disposition: form-data; name="File"; filename="@{triggerBody()?['fileRecord']?['name']}"
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

@{triggerBody()?['file']?['contentBytes']}
--PowerAutomateBoundary--

Questions:

How can we properly construct multipart/form-data body in Power Automate HTTP action?

What is the correct way to attach file content (binary) from trigger or previous step?

Is there any recommended approach or workaround (like connector, Azure Function, etc.) for this scenario?

Any working example or guidance would be really helpful.

Thanks in advance!

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sannavajjala87 388 Super User 2026 Season 1 on at

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Hi,
Yes, this can be done, but multipart/form-data is a little tricky in Power Automate because the HTTP action does not automatically build the multipart body for you.
A few things to check:

Do not manually set only Content-Type: multipart/form-data without a boundary. The boundary in the header must match the boundary used in the body.

Example header:
Content-Type: multipart/form-data; boundary=PowerAutomateBoundary

The body needs to include each form part correctly, including line breaks between headers and values.

Example structure:
--PowerAutomateBoundary
Content-Disposition: form-data; name="schemaName"

schema1
--PowerAutomateBoundary
Content-Disposition: form-data; name="file"; filename="test.xlsx"
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

[file content here]
--PowerAutomateBoundary--

For the file content, make sure you are using the actual file content from a previous action, such as SharePoint Get file content or OneDrive Get file content, not just the file metadata/name.

That said, sending binary files through a manually constructed multipart body can be unreliable in Power Automate, especially if the file content is base64 encoded or treated as text.
The more reliable approach is usually one of these:

Use a custom connector with a properly defined OpenAPI schema for multipart upload

Use an Azure Function as a wrapper API, where Power Automate sends JSON/base64 and the Azure Function converts it to multipart/form-data

If the target API supports it, send the file as base64 JSON instead of multipart/form-data

My recommendation would be to avoid manually building multipart/form-data in the HTTP action unless the payload is very small and simple. For production use, an Azure Function or custom connector wrapper is usually much easier to support and troubleshoot.
Hope this helps.

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Sam_Fawzi 915 Super User 2026 Season 1 on at

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Hey @CU26061342-0

The reason the manual approach keeps failing is that the HTTP action's body is a string, so when you paste contentBytes (base64) into it, you're sending base64 text between the boundaries — not the actual file bytes. The API receives a corrupted file.
The fix is to let the HTTP action build the multipart body for you using its native $multipart schema. Set the HTTP action's body to this:

{ "$content-type": "multipart/form-data", "$multipart": [ { "headers": { "Content-Disposition": "form-data; name=\"schemaName\"" }, "body": "schema1" }, { "headers": { "Content-Disposition": "form-data; name=\"file\"; filename=\"@{triggerBody()?['fileRecord']?['name']}\"", "Content-Type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" }, "body": { "$content-type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", "$content": "@{triggerBody()?['file']?['contentBytes']}" } } ] }

Two important points:

The file part's body is an object with $content-type and $content. When Power Automate sees base64 in $content, it decodes it back to raw binary before sending — which is exactly what the manual string approach can't do.

Don't set the Content-Type: multipart/form-data header yourself. Let the action generate the boundary automatically. Setting it manually is what causes the boundary mismatch.

One note: this JSON body form isn't shown in the visual designer, so you may need to enter it via the code/expression view. It also requires the premium HTTP action.

Hope this helps!

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Vish WR 3,730 on at

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@CU26061342-0

The issue occurs because the HTTP action body is treated as a plain string. When contentBytes (Base64) is pasted into the multipart body, the API receives Base64 text instead of the actual binary file, resulting in a corrupted upload.

Instead, use the HTTP action’s native $multipart schema, which automatically converts the Base64 content back to binary before sending. Also, do not manually set the Content-Type: multipart/form-data header—Power Automate generates the correct boundary automatically.

{ "$content-type": "multipart/form-data", "$multipart": [ { "headers": { "Content-Disposition": "form-data; name=\"schemaName\"" }, "body": "schema1" }, { "headers": { "Content-Disposition": "form-data; name=\"file\"; filename=\"@{triggerBody()?['fileRecord']?['name']}\"", "Content-Type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" }, "body": { "$content-type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", "$content": "@{triggerBody()?['file']?['contentBytes']}" } } ] }

Note: This JSON format may need to be entered using the action’s code/peek view, and it requires the Premium HTTP action.

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