feat: Improve file loading logic in File model (#2861)

The code changes in `files.py` improve the file loading logic in the
File model. The `load()` method now returns the loaded content, which is
then assigned to the `documents` variable. Additionally, the logger now
includes information about the loaded documents.

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

Co-authored-by: Stan Girard <stan@quivr.app>

backend/api/quivr_api/models/files.py

@@ -42,8 +42,10 @@ class File(BaseModel):
         """
         logger.info(f"Computing documents from file {self.file_name}")
         loader = loader_class(self.tmp_file_path)
-        documents = []
-        documents.extend(loader.load())
+        loaded_content = loader.load()
+        documents = (
+            [loaded_content] if not isinstance(loaded_content, list) else loaded_content
+        )
 
         text_splitter = RecursiveCharacterTextSplitter.from_tiktoken_encoder(
             chunk_size=self.chunk_size, chunk_overlap=self.chunk_overlap