refactor(core): simplify role handling and improve usability

- Replaced explicit string usage with `CreatedByRole` enum for better maintainability.
- Removed duplicate `CreatedByRole` class definition, improving codebase consistency.
- Increased file number limits from 6 to 10 to allow more file uploads.
- Transitioned `AppMode` to a string enum for consistent type usage.
- Refactored `extract_thread_messages` function argument for flexibility.
- Removed file extension limitation in file service to support custom extensions.
- Improved enum import statements across multiple modules for clarity and consistency.

api/models/model.py

@@ -29,7 +29,7 @@ class FileUploadConfig(BaseModel):
     allowed_file_types: Sequence[FileType] = Field(default_factory=list)
     allowed_extensions: Sequence[str] = Field(default_factory=list)
     allowed_upload_methods: Sequence[FileTransferMethod] = Field(default_factory=list)
-    number_limits: int = Field(default=0, gt=0, le=6)
+    number_limits: int = Field(default=0, gt=0, le=10)
 
 
 class DifySetup(db.Model):
@@ -40,7 +40,7 @@ class DifySetup(db.Model):
     setup_at = db.Column(db.DateTime, nullable=False, server_default=db.text("CURRENT_TIMESTAMP(0)"))
 
 
-class AppMode(Enum):
+class AppMode(str, Enum):
     COMPLETION = "completion"
     WORKFLOW = "workflow"
     CHAT = "chat"