MIME
What is MIME
Multipurpose Internet Mail Extensions (MIME) is an internet standard created to extend the format of email to support many others in addition to just ASCII text, originally it was only used in SMTP protocol and later on it was extended to support HTTP. MIME type is a meta data set in HTTP request/response header indicating the type of the content. Servers/clients will use this formation to process corresponding content, for example, if the response MIME type is video/ogg, then the browsers can play the video in the browser. In absence of MIME type in the header, browsers will guess the MIME type by looking at the actual content. This is called MIME sniffing. However, different browsers on different platforms may behave differently and there's also security concerns if the content is executable. Note, server can disable MIME sniffing by sending a X-Content-Type-Options along with Content-Type.
Format
MIME types are in type/subtype format. type indicates a general category like text, image, audio, application, multipart, etc. subtype gives more specific information, for example, under text category, there are plain, document, css, etc.
MIME for Web Developers
There are many types of MIME types and some of them are particular important to web developers.
application/octet-stream
application represents a binary file type and application/octet-stream is the default MIME type for binary file, browsers will treat it as a Unknown type and show a download window prompting the user to save it locally.
text/plain
text/plain represents plain text, browsers will display them directly.
text/html
text/html is the type for web pages, a.k.a, document. With this type set, browser will know this is a document and it will parse it and do further processing, like retrieve static files, render style, etc.
application/json
application/json is the MIME type to exchange JSON text. It is often used in XmlHttpRequest requests.
application/x-www-form-urlencoded
application/x-www-form-urlencoded is a MIME type used in HTTP POST request. With this MIME type set, browsers will encode the form data (key/value pairs) into a query string: keyOne=valueOne&keyTwo=ValueTwo. Server will know how to extract the key/value pairs from the query string. For any non-alphanumeric characters, it will be replaced by %HH, each H here is a hexadecimal digit representing the ASCII code of the character.
multipart/form-data
multipart/form-data is another MIME type used in HTTP POST requests. multipart category means the data consists of multiple parts/MIME types. Those different parts are delimited by a boundary (a string started with a double dash '--') and each part is an entity by itself, with its own HTTP headers. For details, see MDN.
application/x-www-form-urlencoded vs multipart/form-data
application/x-www-form-urlencoded and multipart/form-data are the only two MIME types used by HTML form POST, however what are the differences between them and when should we use which?
TL;DR
We should use multipart/form-data if there's any binary data in the form data, otherwise, use application/x-www-form-urlencoded.
As mentioned above, browsers will replace a non-alphanumeric character with 3 characters which will triple the size of the data. With large binary file in form data, tripling the data size will be dramatically inefficient, thus using multipart/form-data will be a better choice. Vice versa, if the form data are small size, using multipart/form-data will introduce much larger overhead (Boundary string).
Another interesting note when using multipart/form-data is the boundary string chosen. The boundary string must be unique from data value, otherwise servers will pick the delimiters in the wrong places. This answer from StackOver provides an interesting option.