Currently if form parameter model contains File field, the content mediatype for this path operation in openapi will be wrong:

from fastapi import FastAPI, Form, File
from pydantic import BaseModel

app = FastAPI()

class MyModel(BaseModel):
    file_model: bytes = File()

@app.post("/file-model")
async def create_item(item: MyModel = Form()):
    return {"file_size": len(item.file_model)}

Request body content mediatype in openapi will be application/x-www-form-urlencoded instead of multipart/form-data:

        "requestBody": {
          "content": {
            "application/x-www-form-urlencoded": {
              "schema": {
                "$ref": "#/components/schemas/MyModel"
              }
            }
          },
          "required": true
        },

This PR enforces the content mediatype to be multipart/form-data for form parameter models that contain at least one File field.
I added if request_media_type == "application/x-www-form-urlencoded" condition to make it possible to override mediatype by passing a parameter to Form:

@app.post("/file-model")
async def create_item(item: MyModel = Form(media_type="whatever")):
    ...