diff --git a/sox/file_info.py b/sox/file_info.py
index 9cb83f8..2998ff0 100644
--- a/sox/file_info.py
+++ b/sox/file_info.py
@@ -326,7 +326,7 @@ def file_extension(filepath):
     extension : str
         The file's extension
     '''
-    return os.path.splitext(filepath)[1][1:]
+    return os.path.splitext(filepath)[1][1:].lower()
 
 
 def info(filepath):

This install of SoX cannot process .MP3 files.
Hi - I am on Ubuntu 18.04. When I try to interact via Python bindings with SoX, I get a warning saying `This install of SoX cannot process .MP3 files.` And then the file is processed just fine. After looking into this issue, I have found that the root cause is in capitalization of file extension that I have on my files.

What do you think about PR to ignore letter size when checking for valid formats? 
diff --git a/tests/test_file_info.py b/tests/test_file_info.py
index d960434..f1ea875 100644
--- a/tests/test_file_info.py
+++ b/tests/test_file_info.py
@@ -12,6 +12,7 @@ def relpath(f):
 SPACEY_FILE = relpath("data/annoying filename (derp).wav")
 INPUT_FILE = relpath('data/input.wav')
 INPUT_FILE2 = relpath('data/input.aiff')
+INPUT_FILE3 = relpath('data/input.WAV')
 EMPTY_FILE = relpath('data/empty.wav')
 INPUT_FILE_INVALID = relpath('data/input.xyz')
 OUTPUT_FILE = relpath('data/output.wav')
@@ -33,6 +34,7 @@ class TestBitrate(unittest.TestCase):
     def test_empty(self):
         actual = file_info.bitrate(EMPTY_FILE)
         expected = None
+        self.assertEqual(expected, actual)
 
 class TestBitdepth(unittest.TestCase):
 
@@ -233,6 +235,11 @@ class TestFileExtension(unittest.TestCase):
         expected = 'x23zya'
         self.assertEqual(expected, actual)
 
+    def test_ext6(self):
+        actual = file_info.file_extension('simplefile.MP3')
+        expected = 'mp3'
+        self.assertEqual(expected, actual)
+
 
 class TestInfo(unittest.TestCase):
 

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest pytest-cov pytest-pep8 numpydoc
apt-get update apt-get install -y gcc sox
3.9
docs/requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.16
babel==2.17.0
certifi==2025.1.31
charset-normalizer==3.4.1
coverage==7.8.0
docutils==0.21.2
exceptiongroup==1.2.2
execnet==2.1.1
idna==3.10
imagesize==1.4.1
importlib_metadata==8.6.1
iniconfig==2.1.0
Jinja2==3.1.6
MarkupSafe==3.0.2
numpy==2.0.2
numpydoc==1.8.0
packaging==24.2
pep8==1.7.1
pluggy==1.5.0
Pygments==2.19.1
pytest==8.3.5
pytest-cache==1.0
pytest-cov==6.0.0
pytest-pep8==1.0.6
requests==2.32.3
snowballstemmer==2.2.0
-e git+https://github.com/rabitt/pysox.git@7166302a18bc6939737f2da6eef5de8d981d799a#egg=sox
Sphinx==7.4.7
sphinxcontrib-applehelp==2.0.0
sphinxcontrib-devhelp==2.0.0
sphinxcontrib-htmlhelp==2.1.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==2.0.0
sphinxcontrib-serializinghtml==2.0.0
tabulate==0.9.0
tomli==2.2.1
urllib3==2.3.0
zipp==3.21.0

name: pysox
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.16
      - babel==2.17.0
      - certifi==2025.1.31
      - charset-normalizer==3.4.1
      - coverage==7.8.0
      - docutils==0.21.2
      - exceptiongroup==1.2.2
      - execnet==2.1.1
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - jinja2==3.1.6
      - markupsafe==3.0.2
      - numpy==2.0.2
      - numpydoc==1.8.0
      - packaging==24.2
      - pep8==1.7.1
      - pluggy==1.5.0
      - pygments==2.19.1
      - pytest==8.3.5
      - pytest-cache==1.0
      - pytest-cov==6.0.0
      - pytest-pep8==1.0.6
      - requests==2.32.3
      - snowballstemmer==2.2.0
      - sphinx==7.4.7
      - sphinxcontrib-applehelp==2.0.0
      - sphinxcontrib-devhelp==2.0.0
      - sphinxcontrib-htmlhelp==2.1.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==2.0.0
      - sphinxcontrib-serializinghtml==2.0.0
      - tabulate==0.9.0
      - tomli==2.2.1
      - urllib3==2.3.0
      - zipp==3.21.0
prefix: /opt/conda/envs/pysox

tests/test_file_info.py::TestFileExtension::test_ext6
tests/test_file_info.py::TestBitrate::test_aiff tests/test_file_info.py::TestBitrate::test_empty tests/test_file_info.py::TestBitrate::test_wav tests/test_file_info.py::TestBitdepth::test_aiff tests/test_file_info.py::TestBitdepth::test_empty tests/test_file_info.py::TestBitdepth::test_wav tests/test_file_info.py::TestChannels::test_aiff tests/test_file_info.py::TestChannels::test_empty tests/test_file_info.py::TestChannels::test_wav tests/test_file_info.py::TestComments::test_aiff tests/test_file_info.py::TestComments::test_empty tests/test_file_info.py::TestComments::test_wav tests/test_file_info.py::TestDuration::test_aiff tests/test_file_info.py::TestDuration::test_empty tests/test_file_info.py::TestDuration::test_spacey_wav tests/test_file_info.py::TestDuration::test_wav tests/test_file_info.py::TestEncoding::test_aiff tests/test_file_info.py::TestEncoding::test_empty tests/test_file_info.py::TestEncoding::test_wav tests/test_file_info.py::TestFileType::test_aiff tests/test_file_info.py::TestFileType::test_empty tests/test_file_info.py::TestFileType::test_wav tests/test_file_info.py::TestNumSamples::test_aiff tests/test_file_info.py::TestNumSamples::test_empty tests/test_file_info.py::TestNumSamples::test_wav tests/test_file_info.py::TestSampleRate::test_aiff tests/test_file_info.py::TestSampleRate::test_empty tests/test_file_info.py::TestSampleRate::test_wav tests/test_file_info.py::TestSilent::test_empty tests/test_file_info.py::TestSilent::test_nonsilent tests/test_file_info.py::TestSilent::test_silent tests/test_file_info.py::TestFileExtension::test_ext1 tests/test_file_info.py::TestFileExtension::test_ext2 tests/test_file_info.py::TestFileExtension::test_ext3 tests/test_file_info.py::TestFileExtension::test_ext4 tests/test_file_info.py::TestFileExtension::test_ext5 tests/test_file_info.py::TestInfo::test_dictionary tests/test_file_info.py::TestValidateInputFile::test_invalid_format tests/test_file_info.py::TestValidateInputFile::test_nonexistent tests/test_file_info.py::TestValidateInputFile::test_valid tests/test_file_info.py::TestValidateInputFile::test_valid_wspaces tests/test_file_info.py::TestValidateInputFileList::test_empty_list tests/test_file_info.py::TestValidateInputFileList::test_invalid_format tests/test_file_info.py::TestValidateInputFileList::test_len_one_list tests/test_file_info.py::TestValidateInputFileList::test_nonexistent tests/test_file_info.py::TestValidateInputFileList::test_nonlist tests/test_file_info.py::TestValidateInputFileList::test_valid tests/test_file_info.py::TestValidateOutputFile::test_file_exists tests/test_file_info.py::TestValidateOutputFile::test_invalid_format tests/test_file_info.py::TestValidateOutputFile::test_not_writeable tests/test_file_info.py::TestValidateOutputFile::test_valid tests/test_file_info.py::TestStat::test_silent_file tests/test_file_info.py::TestStatCall::test_stat_call tests/test_file_info.py::TestParseStat::test_empty tests/test_file_info.py::TestParseStat::test_real_output tests/test_file_info.py::TestParseStat::test_simple
sox/file_info.py
sox/file_info.py