What sorts of files are included in "upload space"?

Looks like I have run out of "upload space". Can you please tell me what sorts of files are included in this? I have deleted numerous forms and submissions in an attempt to make room, but it looks like the space is still exceeded. 

Thank you.

I see that your account is holding Storage subscription plan that lifts the limit of storing 100 submissions in the Free subscription plan only. The free subscription plan allows storing only 100MB of files that are uploaded through your forms. All the files uploaded to the forms will be counted in the storage space. 

I see that your account is currently occupying about 450MB of storage space. In order to clear the storage space of your account, you'll need to delete the submissions from the forms that are having upload fields. If you need to store more files in your account, it is required to upgrade your account to a paid subscription plan.

Hope this information helps!