fix: file loader not picking up uploaded files from API upsert request

[BOOPATB]

Problem

Fixes #6102

When uploading a file via the /api/v1/vector/upsert/:id API endpoint using
multipart/form-data, the file is silently ignored and nothing gets indexed.
This regression was introduced in 3.1.x.

Root Cause

In packages/server/src/utils/upsertVector.ts, the logic that determines
which input field to use for the uploaded file had a copy-paste bug:

// Before (buggy)
} else if (fileInputFieldFromMimeType !== 'txtFile') {
    fileInputField = fileInputFieldFromExt  // ← wrong variable
}

// After (fixed)
} else if (fileInputFieldFromMimeType !== 'txtFile') {
    fileInputField = fileInputFieldFromMimeType  // ← correct variable
}

When a file's extension maps to txtFile but its MIME type correctly
identifies the file type, the code was incorrectly using fileInputFieldFromExt
(which is txtFile) instead of fileInputFieldFromMimeType. This caused the
file to be stored under the wrong input field key, so the File Loader node
never received it.

Changes

• packages/server/src/utils/upsertVector.ts: one-line fix, use
  fileInputFieldFromMimeType instead of fileInputFieldFromExt in the else-if branch

[gemini-code-assist]

Code Review

This pull request fixes a copy-paste error in upsertVector.ts where the incorrect variable was assigned to fileInputField. The review feedback suggests further improving this logic by adding truthiness checks to prevent assigning falsy values, ensuring a more robust fallback to the default field name.

[gemini-code-assist]

While this change correctly fixes the copy-paste error, the logic should also ensure that fileInputField is not assigned a falsy value (such as undefined or null) if the mapping functions fail to return a valid string. Adding a truthiness check provides a more robust fallback mechanism to the default 'txtFile', preventing potential issues with incorrect field mapping in overrideConfig. This implementation uses an if/else if chain, which is preferred for a small number of conditional branches to maintain readability.

Suggested change

	if (fileInputFieldFromExt !== 'txtFile') {
	fileInputField = fileInputFieldFromExt
	} else if (fileInputFieldFromMimeType !== 'txtFile') {
	fileInputField = fileInputFieldFromExt
	fileInputField = fileInputFieldFromMimeType
	}
	if (fileInputFieldFromExt && fileInputFieldFromExt !== 'txtFile') {
	fileInputField = fileInputFieldFromExt
	} else if (fileInputFieldFromMimeType && fileInputFieldFromMimeType !== 'txtFile') {
	fileInputField = fileInputFieldFromMimeType
	}

References

1. Use a default (fallback) implementation unless the specific implementation has meaningfully different behavior or provides better error messages.
2. For a small number of conditional branches, an if/else if chain may be preferred over a map lookup to maintain readability.