Fix prefix handling in bulk file renamer

[Zzy-0608]

Summary

This PR fixes the prefix handling logic in Bulk File Renamer.

Previously, when a user entered a base filename such as photo, the program generated renamed files like _a.txt instead of the expected photo_a.txt. This happened because the conditional expression did not correctly use the user-provided prefix.

Changes

• Fixed the filename prefix generation logic.
• When a base filename is provided, files are renamed using the format prefix_originalName.
• When no base filename is provided, the existing underscore prefix behavior is preserved.
• Improved the renaming logic to make the behavior clearer and easier to maintain.

Testing

Tested locally with sample files:

• a.txt
• b.txt
• c.jpg

Using photo as the base filename, the files are renamed as expected:

• a.txt -> photo_a.txt
• b.txt -> photo_b.txt
• c.jpg -> photo_c.jpg