1. Background#

When I first started writing backend code, if the server return a file (any type), and I would add a Content-Type header and set its value to application/octet-stream:

w.Header().Set("Content-Disposition", "attachment; filename="+strconv.Quote(info.Name()))
w.Header().Set("Content-Type", "application/octet-stream")
http.ServeFile(w, r, filePath)
return

I don’t know what does this mean, but it works. Haha, it’s not a good habit, so I decide to take some time to learn it.

2. MIME types#

A media type (also known as a Multipurpose Internet Mail Extensions or MIME type) indicates the nature and format of a document, file, or assortment of bytes.

2.1. Structure of a MIME type# 

type/subtype

The type represents the general category into which the data type falls, such as video or text.

The subtype identifies the exact kind of data of the specified type the MIME type represents. For example, for the MIME type text, the subtype might be plain (plain text), html (HTML source code), or calendar (for iCalendar/.ics) files.

An optional parameter can be added to provide additional details:

type/subtype;parameter=value

For any MIME type whose main type is text, you can add the optional charset parameter to specify the character set used for the characters in the data. If no charset is specified, the default is ASCII (US-ASCII) unless overridden by the user agent’s settings. To specify a UTF-8 text file, the MIME type text/plain;charset=UTF-8 is used.

There are two classes of type: discrete and multipart: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_Types#types

3. Common MIME types#

3.1. application/octet-stream#

This is the default for binary files. As it means unknown binary file, browsers usually don’t execute it, or even ask if it should be executed. They treat it as if the Content-Disposition header was set to attachment, and propose a “Save As” dialog.

3.2. text/plain#

This is the default for textual files. Even if it really means “unknown textual file,” browsers assume they can display it.

3.3. text/css#

CSS files used to style a Web page must be sent with text/css. If a server doesn’t recognize the .css suffix for CSS files, it may send them with text/plain or application/octet-stream MIME types. If so, they won’t be recognized as CSS by most browsers and will be ignored.

Learn more: Common MIME types - HTTP | MDN

References: MIME types (IANA media types) - HTTP | MDN