Add support for multiple file uploads from a single input element

[Copilot]

Prologue was only capturing the last file when multiple files were uploaded using <input type="file" name="files" multiple />. The FormPart data structure overwrote entries with duplicate names.

Changes

Core data structure (types.nim)

• Changed FormPart.data from OrderedTableRef[string, tuple[...]] to OrderedTableRef[string, seq[tuple[...]]]
• Updated [] operator to return first item for backward compatibility
• Updated []= operator to append instead of replace

Form parser (form.nim)

• Modified parseFormPart to accumulate files with same name into sequences
• Fixed header parsing to reference last-added item with [^1]

API (context.nim)

• Added getUploadFiles(name: string): seq[UploadFile] for retrieving all files
• Existing getUploadFile(name: string) returns first file, unchanged behavior

Usage

proc handleUpload(ctx: Context) {.async.} =
  # New: get all files
  let files = ctx.getUploadFiles("files")
  for file in files:
    file.save("uploads/")
  
  # Old API still works, returns first file
  let file = ctx.getUploadFile("files")

Example in examples/multifile_upload/ demonstrates complete implementation.

Warning

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Original prompt

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This section details on the original issue you should resolve

<issue_title>handle multiple file upload from a single input element</issue_title>
<issue_description>Hello friends, does prologue support retrieving multiple file comming from the same input element?

given the following html form:

<form method="post" action="/uploadFile" enctype="multipart/form-data" >
    <input type="file" name="files" multiple />
    <input type="submit" value="Upload" />
</form>

how do I get the multiple file user uploaded from Context object?

currently ctx.getUploadFile("files") can only get one of the file. Am I missing some other api?

Thanks in advance</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes handle multiple file upload from a single input element #269

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