Fix: Handle missing or empty Accept header in /files/{date}/{file_name} endpoint

[hexboy]

Issue

The endpoint /files/{date}/{file_name} fails with an AttributeError when the Accept header is not provided (e.g., in a simple curl request without headers):

AttributeError: 'NoneType' object has no attribute 'lower'

This occurs because the code assumes accept is always present but does not handle cases where it is None.

Changes

• Added a check for accept is not None before processing the header.
• Made file extension checks from file name if accept is None.
• set ext = None to handle missing/invalid headers gracefully.

Impact

• Fixes: Simple requests (e.g., direct browser access or curl without headers) now work instead of crashing.
• Backward-compatible: Existing requests with valid Accept headers remain unaffected.

Testing

Verified with:

# Without Accept header (previously crashed, now returns the original file)
curl --location 'http://127.0.0.1:8888/files/2025-04-14/abe30891-4656-48de-8cab-fd0d25ea6a34-0.png'  \
  -H 'Accept: '

# With Accept header (unchanged behavior)
curl --location 'http://127.0.0.1:8888/files/2025-04-14/abe30891-4656-48de-8cab-fd0d25ea6a34-0.png' \
  -H 'Accept: image/webp'