While trying to build my own block that allows for a file to be uploaded, I noticed that the add/edit form was not validating properly when no file was selected by the user. I copied this validation code from the built-in file block, so I tried it with the file block as well and realized that it isn't working either (same goes for the flash_content block).
The problem seems to be that the ccmValidateBlockForm function in these blocks' auto.js file are checking for a form element whose id is "ccm-b-file-value", but in fact the appropriate form element id is "ccm-b-file-fm-value". Seems that the html file helper outputs a hidden field that contains the file id, and it appends "-fm-value" to the id of the "choose a file" element, but the javascript validation code (in auto.js) is assuming it's only appending "-value" (not "-fm-value"), and hence it's not finding this DOM element and hence not validating anything.