Fix: Convert 'data' to 'value' for aiohttp.FormData compatibility

[simo665]

The multipart data from handle_message_parameters() uses 'data' as the key, but aiohttp.FormData.add_field() expects 'value'. This causes a TypeError when sending webhooks with file attachments.

Summary

Fixes a bug where sending webhook messages with file attachments fails with:
Example:

if message.attachments:
    files = []
    for att in message.attachments:
        try:
            files.append(await att.to_file())
        except Exception as e:
            pass
    arg["files"] = files

await webhook.send(**arg)

Error

TypeError: FormData.add_field() got an unexpected keyword argument 'data'

Fix: Convert the data key to value before calling add_field().

General Info

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue (please put issue # in summary).
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)