Set file extension filter for form field?

[TechInterMezzo]

Is it possible to set the allowed file extensions for a file form field?
That would help to tell the user what kind of files should be uploaded.
And the file dialog would only show the files with the right extension even when a folder is full of other files.

[r3-gabriel]

It´s not possible in the current feature set. We have a corresponding entry in our own wish list, but it´s currently not prioritized.