Blob 对象表示一个不可变、原始数据的类文件对象。它的数据可以按文本或二进制的格式进行读取，也可以转换成 ReadableStream 来用于数据操作。在 JS 中通常使用 Blob 进行文件下载保存，new 转换过程中需要指定下载文件 MIME 类型。
Blob && MIME
一、介绍
媒体类型（通常称为 Multipurpose Internet Mail Extensions 或 MIME 类型）是一种标准，用来表示文档、文件或字节流的性质和格式。它在IETF RFC 6838中进行了定义和标准化。
互联网号码分配机构（IANA）是负责跟踪所有官方 MIME 类型的官方机构，您可以在 媒体类型 页面中找到最新的完整列表。
二、常见 MIME 类型列表
提示：浏览器通常使用 MIME 类型（而不是文件扩展名）来确定如何处理 URL，因此 Web 服务器在响应头中添加正确的 MIME 类型非常重要。如果配置不正确，浏览器可能会曲解文件内容，网站将无法正常工作，并且下载的文件也会被错误处理。
| 扩展名 | 文档类型 | MIME 类型 |
|---|---|---|
.aac |
AAC audio | audio/aac |
.abw |
AbiWord document | application/x-abiword |
.arc |
Archive document (multiple files embedded) | application/x-freearc |
.avi |
AVI: Audio Video Interleave | video/x-msvideo |
.azw |
Amazon Kindle eBook format | application/vnd.amazon.ebook |
.bin |
Any kind of binary data | application/octet-stream |
.bmp |
Windows OS/2 Bitmap Graphics | image/bmp |
.bz |
BZip archive | application/x-bzip |
.bz2 |
BZip2 archive | application/x-bzip2 |
.csh |
C-Shell script | application/x-csh |
.css |
Cascading Style Sheets (CSS) | text/css |
.csv |
Comma-separated values (CSV) | text/csv |
.doc |
Microsoft Word | application/msword |
.docx |
Microsoft Word (OpenXML) | application/vnd.openxmlformats-officedocument.wordprocessingml.document |
.eot |
MS Embedded OpenType fonts | application/vnd.ms-fontobject |
.epub |
Electronic publication (EPUB) | application/epub+zip |
.gif |
Graphics Interchange Format (GIF) | image/gif |
.htm .html |
HyperText Markup Language (HTML) | text/html |
.ico |
Icon format | image/vnd.microsoft.icon |
.ics |
iCalendar format | text/calendar |
.jar |
Java Archive (JAR) | application/java-archive |
.jpeg .jpg |
JPEG images | image/jpeg |
.js |
JavaScript | text/javascript |
.json |
JSON format | application/json |
.jsonld |
JSON-LD format | application/ld+json |
.mid .midi |
Musical Instrument Digital Interface (MIDI) | audio/midi audio/x-midi |
.mjs |
JavaScript module | text/javascript |
.mp3 |
MP3 audio | audio/mpeg |
.mpeg |
MPEG Video | video/mpeg |
.mpkg |
Apple Installer Package | application/vnd.apple.installer+xml |
.odp |
OpenDocument presentation document | application/vnd.oasis.opendocument.presentation |
.ods |
OpenDocument spreadsheet document | application/vnd.oasis.opendocument.spreadsheet |
.odt |
OpenDocument text document | application/vnd.oasis.opendocument.text |
.oga |
OGG audio | audio/ogg |
.ogv |
OGG video | video/ogg |
.ogx |
OGG | application/ogg |
.otf |
OpenType font | font/otf |
.png |
Portable Network Graphics | image/png |
.pdf |
Adobe Portable Document Format (PDF) | application/pdf |
.ppt |
Microsoft PowerPoint | application/vnd.ms-powerpoint |
.pptx |
Microsoft PowerPoint (OpenXML) | application/vnd.openxmlformats-officedocument.presentationml.presentation |
.rar |
RAR archive | application/x-rar-compressed |
.rtf |
Rich Text Format (RTF) | application/rtf |
.sh |
Bourne shell script | application/x-sh |
.svg |
Scalable Vector Graphics (SVG) | image/svg+xml |
.swf |
Small web format (SWF) or Adobe Flash document | application/x-shockwave-flash |
.tar |
Tape Archive (TAR) | application/x-tar |
.tif .tiff |
Tagged Image File Format (TIFF) | image/tiff |
.ttf |
TrueType Font | font/ttf |
.txt |
Text, (generally ASCII or ISO 8859-n) | text/plain |
.vsd |
Microsoft Visio | application/vnd.visio |
.wav |
Waveform Audio Format | audio/wav |
.weba |
WEBM audio | audio/webm |
.webm |
WEBM video | video/webm |
.webp |
WEBP image | image/webp |
.woff |
Web Open Font Format (WOFF) | font/woff |
.woff2 |
Web Open Font Format (WOFF) | font/woff2 |
.xhtml |
XHTML | application/xhtml+xml |
.xls |
Microsoft Excel | application/vnd.ms-excel |
.xlsx |
Microsoft Excel (OpenXML) | application/vnd.openxmlformats-officedocument.spreadsheetml.sheet |
.xml |
XML | application/xml 代码对普通用户来说不可读 (RFC 3023, section 3) text/xml 代码对普通用户来说可读 (RFC 3023, section 3) |
.xul |
XUL | application/vnd.mozilla.xul+xml |
.zip |
ZIP archive | application/zip |
.3gp |
3GPP audio/video container | video/3gpp audio/3gpp（若不含视频） |
.3g2 |
3GPP2 audio/video container | video/3gpp2 audio/3gpp2（若不含视频） |
.7z |
7-zip archive | application/x-7z-compressed |
参考文档：
- https://developer.mozilla.org/zh-CN/docs/Web/API/Blob
- https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Basics_of_HTTP/MIME_Types
- https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types
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