In an HTML form element, the enctype attribute specifies how the data will be encoded when it is POSTed to the web server. Traditionally, there are two accepted content types for HTML forms:
But there is nothing to stop someone from specifying whatever enctype value that they want (e.g., "application/x-foo"). Of course specifying your own random value doesn't mean that the browser will understand how to submit it or the web server will understand how to process it.
The following lists how some popular browsers respond to unexpected but technically valid enctype values. Each of the MIME types from this list was tested. The following browsers were tested:
Go straight to the conclusions. Or, try some samples yourself.
Version Tested: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/419.3 (KHTML, like Gecko) Safari/419.3
Safari submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| contains the word multipart | multipart/form-data | multipart/example |
| contains the word form-data | multipart/form-data | example/form-data |
| contains the word text | text/plain (content-type specifies nonexistent MIME boundary and posted data is urlencoded) | text/example |
| contains the word plain | text/plain (content-type specifies nonexistent MIME boundary and posted data is urlencoded) | example/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/5.0 (Windows; U; Windows NT 5.1; en) AppleWebKit/522.15.5 (KHTML, like Gecko) Version/3.0.3 Safari/522.15.5
Safari submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| contains the word multipart | multipart/form-data | multipart/example |
| contains the word form-data | multipart/form-data | example/form-data |
| contains the word text | text/plain (content-type specifies empty MIME boundary and posted data is urlencoded) | text/example |
| contains the word plain | text/plain (content-type specifies empty MIME boundary and posted data is urlencoded) | example/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/5.0 (compatible; Konqueror/3.5; Linux) KHTML/3.5.7 (like Gecko)
Konqueror submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| contains the word multipart | multipart/form-data | multipart/example |
| contains the word form-data | multipart/form-data | example/form-data |
| contains the word text | text/plain (content-type specifies nonexistent MIME boundary and posted data is urlencoded) | text/example |
| contains the word plain | text/plain (content-type specifies nonexistent MIME boundary and posted data is urlencoded) | example/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.1.7) Gecko/20070914 Firefox/2.0.0.7
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.7) Gecko/20070914 Firefox/2.0.0.7
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.7) Gecko/20070914 Firefox/2.0.0.7
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Opera/9.23 (Macintosh; PPC Mac OS X; U; en)
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Opera/9.23 (Windows NT 5.1; U; en)
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Opera/9.23 (X11; Linux i686; U; en)
Opera submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; .NET CLR 1.1.4322)
IE submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
IE submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| text/plain | text/plain | text/plain |
| anything else | application/x-www-form-urlencoded | example/example |
Version Tested:
IE submits the following content types:
The following rules are applied based on the value of enctype:
| Specified enctype | Submitted Content-Type | Sample |
|---|---|---|
| (omitted) | application/x-www-form-urlencoded | (omitted) |
| application/x-www-form-urlencoded | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| multipart/form-data | multipart/form-data | multipart/form-data |
| anything else | application/x-www-form-urlencoded | example/example |
As expected, all of the browsers properly support 'application/x-www-form-urlencoded' and 'multipart/form-data' content-type values for the form element's enctype attribute. Additionally, Microsoft Internet Explorer (versions 6 and 7), Mozilla Firefox and Opera support a content-type value of 'text/plain'. The form values are submitted without any encoding or MIME boundary.
Safari and Konqueror exhibit the most idiosyncratic behavior. Both apply a heuristic to determine the proper content-type based on the enctype value by checking for the strings "multipart", "form-data", "text" and "plain". Any enctype that contains "multipart" or "form-data" is treated as 'multipart/form-data'. If "text" or "plain" are found, then the content-type is set to 'text/plain' but the form contents are still urlencoded. An examination of the source code shows the origins of the behavior.
From the KDE source, "kdelibs/khtml/html/html_formimpl.cpp" contains the following:
void HTMLFormElementImpl::setEnctype( const DOMString& type )
{
if(type.string().indexOf("multipart", 0, Qt::CaseInsensitive) != -1 || type.string().indexOf("form-data", 0, Qt::CaseInsensitive) != -1)
{
m_enctype = "multipart/form-data";
m_multipart = true;
m_post = true;
} else if (type.string().indexOf("text", 0, Qt::CaseInsensitive) != -1 || type.string().indexOf("plain", 0, Qt::CaseInsensitive) != -1)
{
m_enctype = "text/plain";
m_multipart = false;
}
else
{
m_enctype = "application/x-www-form-urlencoded";
m_multipart = false;
}
m_encCharset.clear();
}
And from "kdelibs/khtml/khtml_part.cpp":
// construct some user headers if necessary
if (contentType.isNull() || contentType == "application/x-www-form-urlencoded")
browserArgs.setContentType( "Content-Type: application/x-www-form-urlencoded" );
else // contentType must be "multipart/form-data"
browserArgs.setContentType( "Content-Type: " + contentType + "; boundary=" + boundary );
From the WebKit source, "WebCore/html/HTMLFormElement.cpp" contains:
void HTMLFormElement::parseEnctype(const String& type)
{
if(type.contains("multipart", false) || type.contains("form-data", false)) {
m_enctype = "multipart/form-data";
m_multipart = true;
} else if (type.contains("text", false) || type.contains("plain", false)) {
m_enctype = "text/plain";
m_multipart = false;
} else {
m_enctype = "application/x-www-form-urlencoded";
m_multipart = false;
}
}
And from "WebCore/loader/FrameLoader.cpp":
// construct some user headers if necessary
if (contentType.isNull() || contentType == "application/x-www-form-urlencoded")
frameRequest.resourceRequest().setHTTPContentType(contentType);
else // contentType must be "multipart/form-data"
frameRequest.resourceRequest().setHTTPContentType(contentType + "; boundary=" + boundary);
The form element also supports an accept-charset attribute. Additional research may be warranted to determine if different values affect how the data content is encoded and submitted.
The following scripts were used. There are provided in the hope that they may be useful for someone: