Pillow处理BMP图像抛出ValueError: unknown raw mode错误如何解决

使用Pillow库编写图像转base64编码逻辑，用于Flask API调用，该逻辑在多数场景下运行正常，但处理某BMP文件时程序异常崩溃，无明确触发原因。
原有功能实现代码如下：

def get_img_base64(filename):
    # Open Image
    image = Image.open(filename)
    # Saving the format
    format = image.format
    buffered = BytesIO()
    image.save(buffered, format)
    img_str = base64.b64encode(buffered.getvalue())
    return "data:image/" + format + ";base64," + img_str.decode()

程序运行时抛出的完整错误栈如下：

Traceback (most recent call last):
  File "test.py", line XXX, in XXX
    imgBase64 = get_img_base64(path)
  File "test.py", line XXX, in get_img_base64
    image.save(buffered, format)
  File "python3.10/site-packages/PIL/Image.py", line 2264, in save
    self._ensure_mutable()
  File "python3.10/site-packages/PIL/Image.py", line 626, in _ensure_mutable
    self._copy()
  File "python3.10/site-packages/PIL/Image.py", line 619, in _copy
    self.load()
  File "python3.10/site-packages/PIL/ImageFile.py", line 234, in load
    err_code = decoder.decode(b"")[1]
  File "python3.10/site-packages/PIL/BmpImagePlugin.py", line 326, in decode
    self.set_as_raw(bytes(data), ("P", 0, self.args[-1]))
  File "python3.10/site-packages/PIL/ImageFile.py", line 684, in set_as_raw
    d = Image._getdecoder(self.mode, "raw", (rawmode))
  File "python3.10/site-packages/PIL/Image.py", line 435, in _getdecoder
    return decoder(mode, *args + extra)
ValueError: unknown raw mode for given image mode

已尝试参考通用解决方案将图像转换为"P"模式，但转换过程中依然抛出ValueError: unknown raw mode for given image mode错误。

执行exiftool -v IMG.BMP命令获取的文件元信息如下，确认该文件为8位色深、采用RLE压缩（Compression=1）的BMP格式：

ExifToolVersion = 12.40
  FileName = IMG.BMP
  Directory = .
  FileSize = 44978
  FileModifyDate = 1655391432
  FileAccessDate = 1655391514
  FileInodeChangeDate = 1655454712
  FilePermissions = 33272
  FileType = BMP
  FileTypeExtension = BMP
  MIMEType = image/bmp
  + [BinaryData directory, 40 bytes]
  | BMPVersion = 40
  | ImageWidth = 594
  | ImageHeight = 163
  | Planes = 1
  | BitDepth = 8
  | Compression = 1
  | ImageLength = 43900
  | PixelsPerMeterX = 2833
  | PixelsPerMeterY = 2833
  | NumColors = 256
  | NumImportantColors = 256

报错根源是旧版本Pillow对采用RLE8压缩的8位BMP存在已知解码兼容bug：该异常文件是BI_RLE8压缩的8位索引色BMP，Pillow自带的BmpImagePlugin解码这类文件时，传给raw解码器的调色板模式参数不匹配，直接触发未知raw mode错误。手动转P模式无法解决问题，因为错误发生在图像加载阶段，还没等执行模式转换，解码流程就已经崩溃。

两种方案按需选择即可：

• 方案一：直接升级Pillow到最新版本。目前新版Pillow已经修复了绝大多数RLE8压缩BMP的解码问题，执行命令pip install --upgrade pillow完成升级后，原有代码无需修改即可正常处理这类文件。
• 方案二：如果受环境限制无法升级Pillow版本，可绕开P模式的解码逻辑，加载图像后先强制转换为RGB模式再做内存存储，转码时优先用PNG格式避免BMP编码的兼容问题，修改后的可直接复用的代码如下：

import base64
from io import BytesIO
from PIL import Image

def get_img_base64(filename):
    image = Image.open(filename)
    save_format = image.format
    # 针对RLE压缩的8位BMP做兼容处理
    if image.format == "BMP" and image.mode == "P":
        image = image.convert("RGB")
        save_format = "PNG"
    buffered = BytesIO()
    image.save(buffered, save_format)
    img_str = base64.b64encode(buffered.getvalue())
    return f"data:image/{save_format.lower()};base64,{img_str.decode()}"

注意：转RGB后如果强行保存为BMP格式，部分旧版本Pillow仍可能抛出编码错误，内存中转选择PNG格式即可，最终生成的base64不管是用于前端渲染还是接口传参，兼容性和原BMP没有差异。

内容的提问来源于stack exchange，提问作者PythonNewbie