form.d: rephrased somewhat, added two example command lines

For SMTP and IMAP protocols, this is the mean to compose a multipart mail

message to transmit.

This enables uploading of binary

files etc. To force the 'content' part to be a file, prefix the file name with

an @ sign. To just get the content part from a file, prefix the file name with

the symbol <. The difference between @ and < is then that @ makes a file get

attached in the post as a file upload, while the < makes a text field and just

get the contents for that text field from a file.

Example: to send an image to an HTTP server, where \&'profile' is the name of

the form-field to which portrait.jpg will be the input:

This enables uploading of binary files etc. To force the 'content' part to be

a file, prefix the file name with an @ sign. To just get the content part from

a file, prefix the file name with the symbol <. The difference between @ and <

is then that @ makes a file get attached in the post as a file upload, while

the < makes a text field and just get the contents for that text field from a

file.

Tell curl to read content from stdin instead of a file by using - as

filename. This goes for both @ and < constructs. When stdin is used, the

contents is buffered in memory first by curl to determine its size and allow a

possible resend.  Defining a part's data from a named non-regular file (such

as a named pipe or similar) is unfortunately not subject to buffering and will

be effectively read at transmission time; since the full size is unknown

before the transfer starts, such data is sent as chunks by HTTP and rejected

by IMAP.

Example: send an image to an HTTP server, where \&'profile' is the name of the

form-field to which the file portrait.jpg will be the input:

 curl -F profile=@portrait.jpg https://example.com/upload.cgi

To read content from stdin instead of a file, use - as the filename. This goes

for both @ and < constructs. If stdin is not attached to a regular file, it is

buffered first to determine its size and allow a possible resend. Defining a

part's data from a named non-regular file (such as a named pipe or similar) is

unfortunately not subject to buffering and will be effectively read at

transmission time; since the full size is unknown before the transfer starts,

data is sent as chunks by HTTP and rejected by IMAP.

Example: send a your name and shoe size in two text fields to the server:

 curl -F name=John -F shoesize=11 https://example.com/

Example: send a your essay in a text field to the server. Send it as a plain

text field, but get the contents for it from a local file:

 curl -F "story=<hugefile.txt" https://example.com/

You can also tell curl what Content-Type to use by using 'type=', in a manner

similar to: