Document multipart serializer mislabels MIME and emits duplicate file parts

Summary

The Java SDK's document multipart serializer has two independent wire-format defects in one request:

1. Runtime JPG/JPEG/PNG/JFIF paths are compared with ==, so a normal proof.png path is classified as image/pdf. PDF also falls through to the non-standard image/pdf rather than documented application/pdf.
2. The serializer adds file once as a binary part, then iterates the complete request object and adds the same file key again as a text pathname.

Affected commit: ad9ab7b6e6f045b782dfd7608da04de9f930ad97

Reproduction

I executed the pinned official SDK serializer through reflection and wrote the resulting RequestBody to an Okio buffer:

{
  "png_media_type": "image/pdf",
  "file_parts": 2
}

Input shape:

{
  "file": "/tmp/proof.png",
  "purpose": "dispute_evidence"
}

Relevant source is src/main/java/com/razorpay/ApiUtils.java:

• getMediaType compares the substring extension using == and |.
• fileRequestBody adds the binary file part.
• Its following loop adds every request key, including file, again as a text part.

The Document API documents a singular file field and MIME values image/jpg, image/jpeg, image/png, and application/pdf.

Expected

• One binary multipart part named file.
• Correct documented MIME type for the selected file.
• Unknown extensions rejected rather than classified as PDF.

Suggested fix

• Map lower-cased extensions with .equals/.equalsIgnoreCase to their exact MIME types.
• Exclude file from the ordinary form-field loop.
• Add a multipart wire snapshot test asserting both MIME and field cardinality.

No credentials or provider-side mutation are required to reproduce this SDK serialization defect.

Start in src/main/java/com/razorpay/ApiUtils.java, reading getMediaType and fileRequestBody. Reproduce the serializer output for the PNG and PDF cases, then add or update a multipart wire test covering MIME values, unknown extensions, and file-part cardinality. Done means one binary file part uses the documented MIME type and ordinary fields do not duplicate file.