In HTML, a file-select control is a component of a web form with which a user can select a file from his local machine. When the form is submitted (perhaps together with other form data), the file is uploaded to the web server. There, when the file arrives, some action usually takes place, such as saving the file on the web server. However, the particular action that takes place is determined by the server-side script to which the form is submitted.
Here is a code example of a web form with a file-select control. It is the input element with type="file" that creates the file-select control.
When it comes to the rendering on the screen of a file-select control, there is some variation among web browsers. Typically, on a windows-based platform, user agents will render a file-select control as a text field, together with a "Browse" button. When the "Browse" button is pressed, a file dialog opens, with which actual file selection on one's platform can take place. After selection, the filename of the selected file is displayed in the text field, with its full path. Alternatively, instead of using the "Browse" button, the filename can be entered directly in the text field.
The mechanism for form-based file upload was originally proposed in RFC 1867 (published November 1995), as an extension to HTML 2.0 (RFC 1866), after its publication. Form-based file upload then was incorporated in HTML 3.2, which explicitly refers to RFC 1867 for further information on form-based file upload.
HTML 4.01 does not, in itself, describe how the file-select control is supposed to work, but it does list RFC 2388 and RFC 1867 as references.
Multiple file selection
The intention in RFC 1867 is that a single file-select control should allow selection of multiple files. This intention seems reflected in HTML 4.01, which, for the file-select control- type, states
This control type allows the user to select files so that their contents may be submitted with a form. The INPUT element is used to create a file select control.
It has been noted that the plural "files" in the above quote is an indication that, in HTML 4.01, a single-file select-control still was supposed to handle selection of multiple files and not just a single file.
RFC 1867 also introduced the accept attribute for the input element. This would enable file-type filtering based on MIME type for the file-select control.
In addition, it is proposed that the INPUT tag have an ACCEPT attribute, which is a list of comma-separated media types.
If an ACCEPT attribute is present, the browser might constrain the file patterns prompted for to match those with the corresponding
appropriate file extensions for the platform.
Thus, a user-agent may restrict file selection, as, for example, in the following, restricted to GIF and PNG images:
On a Windows platform, this might mean that the user agent would show files only of the types specified in the browse-file dialog.
Basic support for the file-select control was adopted quickly by browser vendors. For example, already Internet Explorer 4, Netscape Navigator 2.0 and Opera 3.5 recognized the input element of type="file" as a file-select control.
However, most modern browsers still do not implement the file-select control as it was intended, or lack certain features.
Cannot select multiple files
Generally, support is lacking for form-based upload of multiple files with a single file-select control. One source states that Opera supports multiple-file selection through a single file-select control. This was true for Opera versions starting from 3.5, in which the file-upload feature was introduced. However, with the first beta release of Opera 7 line, this function was no longer available.
Accept attribute has no effect
Most, if not all, browsers make no use of the accept attribute.
The lack of support for multiple-file selection has led developers to search for alternative solutions. These solutions have in common that they do not use the input type="file" element.
One solution is to use a Java Applet. An example of this is JUpload It allows selection of multiple files within its file browser as well as offering a file-type filter.
An example of a Flash-based file-upload utility is FancyUpload, which is based on MooTools. It too allows selection of multiple files as well as offering a file-type filter.