_UPLOAD_SENTENCE_RE leaves garbled text when filename contains periods

Description

DeerFlow's upload sentence regex fails to cleanly remove sentences containing filenames with periods. The regex matches up to the period in the filename extension, leaving a garbled fragment (`'pdf. They asked about Python.'`) after the sentence replacement.

Reproduction

# Input: 'The user uploaded a file called report.pdf. They asked about Python.'
# After regex substitution: 'pdf. They asked about Python.'
# Garbled fragment left behind