`from_buffer()` issue with file-5.47

[shosca]

Using archlinux, with python-magic 0.4.27, there seems to be an issue with file-5.47-1 and python-magic when using from_buffer

In [1]: import magic

In [2]: magic.from_file("sample.mp4", mime=True)
Out[2]: 'video/mp4'

In [3]: magic.from_buffer(open("sample.mp4", "rb").read(2048), mime=True)
Out[3]: 'application/octet-stream'

In [4]: magic.from_file("sample.webm", mime=True)
Out[4]: 'video/webm'

In [5]: magic.from_buffer(open("sample.webm", "rb").read(2048), mime=True)
Out[5]: 'application/octet-stream'

after downgrading to file-5.46-5 everything works

In [1]: import magic

In [2]: magic.from_file("sample.mp4", mime=True)
Out[2]: 'video/mp4'

In [3]: magic.from_buffer(open("sample.mp4", "rb").read(2048), mime=True)
Out[3]: 'video/mp4'

In [4]: magic.from_file("sample.webm", mime=True)
Out[4]: 'video/webm'

In [5]: magic.from_buffer(open("sample.webm", "rb").read(2048), mime=True)
Out[5]: 'video/webm'

[ddelange]

I can confirm this issue, just tried bumping to file-5.47 in #294 and the smoke test starts failing:

  Successfully installed python-magic-0.4.28
  + /bin/sh -c 'python -c "import magic; assert magic.Magic(mime=True).from_buffer(b'"'"'\x00\x00\x00\x1cftypiso5\x00\x00\x00\x01isomiso5hlsf\x00\x00'"'"') == '"'"'video/mp4'"'"'"'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  AssertionError

Sounds like an upstream issue because this library is just a CDLL wrapper.

[bvergnaud]

Hiya,

For anyone stumbling here, a similar problem was reported upstream here (requires creating an account).

Date Submitted: 2026-03-04 12:58
Last Update: 2026-04-01 08:54

They seem to be on the right track. I spent some time digging tonight, I had identified a commit that messed with offset calculations in file_buffer(). I intended to report it, but I found that issue, and based on the latest comments from today, they found the same commit and code paths I was looking at.

In the meantime:

• Using magic.from_file() or magic.from_descriptor() works fine.
• If you really want to use buffers (e.g. to avoid reading huge files), save the buffer to a temporary file and use magic.from_file() on that.
• Or pin your file package ≤ 5.46, until further news.

---

UPDATE: The bug is reported as patched by one of the maintainers, as of 2026-04-12 22:15 (UTC). All we need now is to wait for a 5.48 release. Which unfortunately might take a while: 5.47 landed Feb 26, 2026, 5.46 landed Nov 27, 2024. Let's hope we won't have to wait 1 more year. 😅

[doronbehar]

Did anyone manage to figure out what upstream commit fixed the issue? I didn't find a clear candidate here:

• https://bugs.astron.com/view.php?id=725

I'd like to patch it for my distribution...

[bvergnaud]

Did anyone manage to figure out what upstream commit fixed the issue? I didn't find a clear candidate here:

* https://bugs.astron.com/view.php?id=725

I'd like to patch it for my distribution...

Here you go:
file/file@c546057

[RafayAK]

Just faced this issue, for somereason audio/ogg is getting picked up as application/octet-stream

 python -u - <<'PY'
import magic
p='/Path/To/Audio-Notes/Q1.ogg'
m=magic.Magic(mime=True)
print('from_file', m.from_file(p))
with open(p,'rb') as f:
    b=f.read(8192)
print('from_buffer_2048', m.from_buffer(b[:2048]))
print('from_buffer_8192', m.from_buffer(b))
PY
from_file audio/ogg
from_buffer_2048 application/octet-stream
from_buffer_8192 application/octet-stream