chore Use pdf library to check file without extension

[yuming-long]

Summary

Instead of manually checking filename with .pdf extension and return is_pdf = false -> use currently pdf library to read file content to decide if the file content is valid pdf

[awalker4]

LGTM! I'd just suggest removing the logging when a file can't be loaded in pypdf. For a .doc, I see this:

INFO: Preparing to split document for partition.
ERROR: 'utf-8' codec can't decode byte 0xd0 in position 0: invalid continuation byte
WARNING: The file does not appear to be a valid PDF.
INFO: Partitioning without split.

If any non pdf is expected to hit this path, we can remove the scary ERROR and WARNING.

[awalker4]

LGTM pending the last comment!