What hook gets called at the formation of a form? (hook_form_alter called for adjusting a form) what hook gets called at the formation of a form.

I'm trying to add some additional mime types to form with id file_entity_add_upload but my hook_form_alter is getting called after these are validated, so attempts to upload them fails.

Update - added screenshot to illustrate the problem



Update 2
the mime types list in file_entity_get_upload_validators function in file_entity.pages.inc dictates what can be uploaded - see this screenshot



Specifically, in, file_entity_get_upload_validators, the line that populates the valid mime types with hard-coded values is (my screenshot didn't completely show the line): 
$validators['file_validate_extensions'] = array(variable_get('file_entity_default_allowed_extensions', 'jpg jpeg gif png txt doc docx xls xlsx pdf ppt pptx pps ppsx odt ods odp'));

Update 3

It would appear that the actual checking of the mime type happens in core module "file", modules/file/file.js : if (!acceptableMatch.test(this.value)) {
        var error = Drupal.t("The selected file %filename cannot be uploaded. Only files with the following extensions are allowed: %extensions.", {
 I searched the error message shown in following screenshot in the code and file.js was a result - which looks like where the actual validation is taking place.


New error with Andy's answer - maybe it's me will look into it a little later.


Raised as an issue here:
https://www.drupal.org/node/2295671
There's this brilliant image on the Form API Internal Workflow in drupal.org, you can see the build process and structure. You've got callbacks such as #process and #after_build that could be useful for your use case.