i use a plugin (Formidable Pro) to build forms. The upload field uses the core functionality of WP (the same you can use to add Media to Library).
Opening the form from a mobile device has strange behaviour when selecting file to upload.
From an Android device (using the standard included browser and tried with Dolphin HD browser too) it allows to select file eg. from gallery of the smartphone but when done it simply reload the page and the file is not selected.
From an iOS device (iPhone 4) when press the button for choosing file simple does nothing.
Im sure that with native WordPress iOS/Android app the upload works well but naturally i can't ask mobile users to install it, they should be able to upload files via the form from the web browser.
Thanks in advice