Fix form files when request buffering is enabled

[kiryl-labada]

Fix form files when request buffering is enabled

When request buffering is enabled, processing files in parallel throws an exception

Description

When request body buffering is enabled, form files share a single underlying buffering stream. This causes a problem because a Stream cannot be read in parallel. When buffering is disabled, this issue does not occur, because each file is buffered separately.

The most straightforward solution is to buffer files regardless of how the request body is handled. However, to avoid negatively affecting performance for requests that contain only one file, the following optimization is applied:

• The first FormFile references the request body stream directly (no memory or disk allocation).
• All subsequent files are buffered.

Fixes #64843

[kiryl-labada]

@dotnet-policy-service agree