Fix: Provide clear error when filename is missing on /data endpoint (Fixes #220)

[Deekshit014]

Proposed changes

Please include a summary of the changes here and why we need those changes. And also let us know which issue is fixed.

This Pull Request resolves Issue #220.

The issue was that when a developer attempted to upload data to the /data endpoint using the multipart/form-data content type but omitted the required filename, the API returned a generic or confusing validation error.

The change introduces an explicit check at the start of the form upload handler to ensure the file part and filename are present, returning a clear, user-friendly error message (MsgFilenameRequired) instead of letting the request fail later with a misleading error.

Fixes #220

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Types of changes

• Bug fix
• New feature added
• Documentation Update

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Please make sure to follow these points

• I have read the contributing guidelines.
• I have performed a self-review of my own code or work.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generates no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• My changes have sufficient code coverage (unit, integration, e2e tests).

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Screenshots (If Applicable)

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Other Information

The fix was applied in internal/apiserver/route_post_data.go within the CoreFormUploadHandler. I inserted the check for r.Part == nil || r.Part.FileName == "" at the start of the handler to ensure the error is returned at the point of validation, leading to better developer experience.