docs: add mime_type to PDF file content examples (url/base64 only)

Add explicit mime_type: "application/pdf" to URL and base64 PDF document
input examples in the messages documentation. This clarifies that
mime_type is required when using type: "file" content blocks with
url or base64 sources, since providers don't infer MIME type from these.

Note: file ID (source_type: "id") examples do NOT need mime_type since
providers already know the MIME type from when the file was uploaded.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: chunghyunleeme

src/oss/langchain/messages.mdx

@@ -906,7 +906,11 @@ for your chosen model for specifics.
         "role": "user",
         "content": [
             {"type": "text", "text": "Describe the content of this document."},
-            {"type": "file", "url": "https://example.com/path/to/document.pdf"},
+            {
+                "type": "file",
+                "url": "https://example.com/path/to/document.pdf",
+                "mime_type": "application/pdf",
+            },
         ]
     }
 
@@ -1024,7 +1028,7 @@ for your chosen model for specifics.
     const message = new HumanMessage({
       content: [
         { type: "text", text: "Describe the content of this document." },
-        { type: "file", source_type: "url", url: "https://example.com/path/to/document.pdf" },
+        { type: "file", source_type: "url", url: "https://example.com/path/to/document.pdf", mime_type: "application/pdf" },
       ],
     });
 
@@ -1036,6 +1040,7 @@ for your chosen model for specifics.
           type: "file",
           source_type: "base64",
           data: "AAAAIGZ0eXBtcDQyAAAAAGlzb21tcDQyAAACAGlzb2...",
+          mime_type: "application/pdf",
         },
       ],
     });