There's been a handful of reports where "obviously" not a UDIM filenames
were detected as such during image open.[1]

This change makes the detection less aggressive by enforcing that the
4-digit sequence be delineated on both sides by one of the following 3
characters ., -, _

This fixes the problem for such filenames as:
"screenshot-1080p.png", "Image-1920x1080.png", "(1999) Photo.png", and
"antiguaChestnut_X_1240Wx814H.png"

[1] T97366 T98918 T99154 T100035

Some filenames remain problematic like "IMG_1234.png" or similar.