What is the problem you are trying to solve?
The file input field allowes just to select one file a time. But more and more applications will require to upload more than one file. It would be very helpful for the user to be able to select multiple files in one field.
What is the feature you are suggesting to help solve it?
An additional attribute for the field. For example multiple="multiple". The files will be uploaded the same way like in todays browsers with several file input fields, but all files will be uploaded having the same field name. But if you use something like "file[]" as the name, popular scripting-languages like PHP will accept them without any modification to the language itself.
Browser without support for this feature would just ignore this attribute, allowing to upload just one file like today. That is acceptable.
I also could think about some additional, but optional attributes like "max-files" (defining the maximal possible files accepted by the application) or "rows". If rows has a bigger value than 1, the field would be displayed like a textarea with one file per row. If more files are displayed, a scrollbar would be presented. If rows is set to its default value 1, it would look like todays file inputs and the files would be seperated by a coma or semicolon.
Why do you think browsers would implement this feature?
Because it is easy to do without much side effects. Also it greatly helps the users.
Why do you think authors would use this feature?
Because it is often implemented by many webdevelopers using flash or javascript.
What evidence is there that this feature is desparately needed?
The wide usage of plugins in flash or java implementing the feature over and over again.