how to search for a file type in linux code example

Example 1: find linux with extension

find /home/username/ -name "*.err"

Example 2: linux find file

# syntax 
# find *</path/to/dir> '(' *<Filter-1> *<and/or-operator> *<filter-2> ')'

# example 
find . '(' -name "Test*.pm" -a ! -name 'Useless*' ')'