Feat: File uploads for future conversations on SDK API (#13378)

### What problem does this PR solve?

This PR aims to:

1. Enable file uploads for the public API, similarly to what
/document/upload_info accomplishes for the frontend;
2. Enable files sent to the /chat/:chat_id/completions endpoint to be
used within the conversation.
We classify the first item as a new future, while classifying the second
one as a bug fix.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

*The work related to this PR was co-authored by*

[Bruno Ferreira](https://github.com/brunopferreira): Custom Solutions
Manager @ [Orbcom](https://orbcom.pt/)
[Pedro Ferreira](https://github.com/sirj0k3r): Lead Software Developer @
[Orbcom](https://orbcom.pt/)
[Pedro Cardoso](https://github.com/pedromiguel4560): Associate Software
Developer @ [Orbcom](https://orbcom.pt/)

*This PR replaces #13248*

---------

Co-authored-by: Pedro Cardoso <pedrocardoso@orbcom.pt>
Co-authored-by: Pedro Ferreira <pedroferreira@orbcom.pt>

api/apps/sdk/files.py

@@ -148,6 +148,62 @@ async def upload(tenant_id):
         return server_error_response(e)
 
 
+@manager.route("/file/upload_info", methods=["POST"])  # noqa: F821
+@token_required
+async def upload_info(tenant_id):
+    """
+    Upload runtime file metadata for SDK chat completions.
+    ---
+    tags:
+      - File
+    security:
+      - ApiKeyAuth: []
+    parameters:
+      - in: formData
+        name: file
+        type: file
+        required: false
+        description: File(s) to upload as runtime attachments.
+      - in: query
+        name: url
+        type: string
+        required: false
+        description: Optional URL to fetch and convert into a runtime attachment.
+    responses:
+      200:
+        description: Runtime attachment descriptor(s) for the `files` field in completions requests.
+    """
+    files = await request.files
+    file_objs = files.getlist("file") if files and files.get("file") else []
+    url = request.args.get("url")
+
+    if file_objs and url:
+        return get_json_result(
+            data=False,
+            message="Provide either multipart file(s) or ?url=..., not both.",
+            code=RetCode.BAD_REQUEST,
+        )
+
+    if not file_objs and not url:
+        return get_json_result(
+            data=False,
+            message="Missing input: provide multipart file(s) or url",
+            code=RetCode.BAD_REQUEST,
+        )
+
+    try:
+        if url and not file_objs:
+            return get_json_result(data=FileService.upload_info(tenant_id, None, url))
+
+        if len(file_objs) == 1:
+            return get_json_result(data=FileService.upload_info(tenant_id, file_objs[0], None))
+
+        results = [FileService.upload_info(tenant_id, f) for f in file_objs]
+        return get_json_result(data=results)
+    except Exception as e:
+        return server_error_response(e)
+
+
 @manager.route('/file/create', methods=['POST'])  # noqa: F821
 @token_required
 async def create(tenant_id):