[csharp][generichost] Better file support

[devhl-labs]

Fix MultipartFormDataContent Generation for File Uploads in C# GenericHost

Summary

This PR fixes critical issues with multipart file upload code generation in the C# GenericHost library. The generated code now correctly handles single files, arrays of files, and mixed form parameters.

Closes

• [BUG][CSHARP][GENERICHOST] - Array of string/binary for multi-file uploads generates invalid code #21380 - Invalid C# code generation for endpoints with file uploads
• [csharp][generichost] Multi file uploads #21449 - Multipart content handling improvements

Changes

Issues Fixed

1. Content-Type Header: Changed from MultipartContent (multipart/mixed) to MultipartFormDataContent (multipart/form-data) to match OpenAPI specifications

2. Array of Files Support: Added proper iteration logic for file parameters that are arrays/containers

   • Single files are added directly to multipart content
   • File arrays are iterated and each file is added individually

3. Proper Field Names: Files now use their actual parameter names instead of hardcoded values, allowing servers to correctly parse form fields

4. Conditional FormUrlEncodedContent: Only adds FormUrlEncodedContent when there are actual non-file form parameters, preventing empty content additions

5. Content Assignment Order: httpRequestMessageLocalVar.Content is now assigned after the multipart content is fully populated, improving code clarity and reliability

Generated Code Behavior

Before:

MultipartContent multipartContentLocalVar = new MultipartContent();
httpRequestMessageLocalVar.Content = multipartContentLocalVar;
multipartContentLocalVar.Add(files);  // Adds entire collection as single content
multipartContentLocalVar.Add(new FormUrlEncodedContent(emptyList));  // Unnecessary

After:

MultipartFormDataContent multipartContentLocalVar = new MultipartFormDataContent();
foreach (var file in files)
{
    var streamContentLocalVar = new StreamContent(file);
    multipartContentLocalVar.Add(streamContentLocalVar, "fieldName");
}
if (formParameters.Count > 0)
    multipartContentLocalVar.Add(new FormUrlEncodedContent(formParameters));
httpRequestMessageLocalVar.Content = multipartContentLocalVar;

Testing

Generated code now:

• ✅ Compiles without errors for file upload endpoints
• ✅ Correctly handles single file parameters
• ✅ Correctly handles array of files parameters
• ✅ Properly sets multipart/form-data content type
• ✅ Includes correct field names for form parsing
• ✅ Only adds form parameters when necessary

Files Modified

• modules/openapi-generator/src/main/resources/csharp/libraries/generichost/api.mustache - Updated file parameter handling in request building logic

PR checklist

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  Commit all changed files.
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Summary by cubic

Adds correct multipart/form-data handling and multi-file upload support to C# GenericHost clients. Fixes broken file parameter generation and updates tests/samples to validate single and multiple file uploads. Fixes #21380 and #21449.

• New Features

  • Support arrays/collections of files by iterating and adding each file with the correct field name.
  • Added a new spec endpoint (/pet/{petId}/uploadImages) and regenerated C# samples to exercise multi-file uploads.

• Bug Fixes

  • Use MultipartFormDataContent instead of MultipartContent to send multipart/form-data.
  • Add FormUrlEncodedContent only when non-file form fields are present.
  • Set field names from parameter names.

^{Written for commit ff5bd0b. Summary will update on new commits.}