feat: add file origin type

[fatelei]

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Summary

fix #31099

in second conversation

• image type first is custom
• second, use the build_from_message_file the type become image, so the check _is_file_valid_with_config fail

Screenshots

Before	After
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before

Screen.Recording.2026-01-19.at.10.48.48.mov

after

Screen.Recording.2026-01-19.at.10.44.42.mov

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug where file type validation would fail in multi-turn conversations. The issue arose because a file initially classified as 'custom' could be re-evaluated to a more specific type (e.g., 'image') during subsequent processing, causing validation checks to incorrectly reject it. The solution involves introducing an original_type field to the File model, which preserves the file's initial type. This original_type is then used for validation and serialization, ensuring that files are consistently and correctly processed throughout their lifecycle within the application, particularly in conversational contexts.

Highlights

• Introduction of original_type field: A new original_type field has been added to the File model to store the initial, user-specified file type, preventing its loss during subsequent processing or re-evaluation.
• Preservation of original file type during serialization: The File model's serialization logic (to_dict and a new _serialize_model method) is updated to prioritize the original_type when outputting the file's type, ensuring that the original intent is reflected in serialized data.
• Conditional file type determination in build_from_message_file: Logic has been added to build_from_message_file to conditionally set the file's type to FileType.CUSTOM if the configuration only allows custom types or specifies allowed extensions. Otherwise, it defaults to the type from the message file.
• Validation using original_type: The file validation process, specifically _is_file_valid_with_config, now checks against the original_type if available, falling back to the current type field. This resolves issues where file types were incorrectly re-evaluated, leading to validation failures.
• Propagation of original_type in file factory methods: Various file creation methods within file_factory.py (e.g., _build_from_local_file, _build_from_remote_url, _build_from_tool_file, _build_from_datasource_file) have been updated to correctly capture and set the original_type when constructing File objects.

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[gemini-code-assist]

Code Review

This pull request introduces an original_type field to the File model to preserve the file type specified during upload, which is a good approach to fix the validation issue. The changes are logical and correctly address the problem. I've left a few comments with suggestions to improve code quality by removing redundancy and simplifying logic. Overall, this is a good contribution.