fix: apply extension-based MIME validation when useTempFiles is enabled

When `useTempFiles` is true, `file.data` is an empty buffer so
`fileTypeFromBuffer` returns undefined. The fallback extension-based
validation was gated behind `!useTempFiles`, which meant it was skipped
entirely — allowing any file type to bypass MIME restrictions.

Remove the `!useTempFiles` condition so extension-based validation
always runs when buffer detection fails, regardless of temp file mode.

Fixes #16233

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: eddieran

packages/payload/src/uploads/checkFileRestrictions.ts

@@ -53,7 +53,6 @@ export const checkFileRestrictions = async ({
 }: checkFileRestrictionsParams): Promise<void> => {
   const errors: string[] = []
   const { upload: uploadConfig } = collection
-  const useTempFiles = req?.payload?.config?.upload?.useTempFiles ?? false
   const configMimeTypes =
     uploadConfig &&
     typeof uploadConfig === 'object' &&
@@ -105,7 +104,7 @@ export const checkFileRestrictions = async ({
       }
     }
 
-    if (!detected && !useTempFiles) {
+    if (!detected) {
       const mimeTypeFromExtension = getFileTypeFallback(file.name).mime
       const extIsValid = validateMimeType(mimeTypeFromExtension, configMimeTypes)