Describe the bug
In the proposal form (create/edit) there are 2 areas: Add image and Add documents. On the Add image field any user can add a .pdf file without any restriction and after finalizing all the creation steps and going to "edit proposal", that .pdf will appear on "add documents" field.

To Reproduce
Steps to reproduce the behavior:

1. Enable attachments for component
2. Go to "New proposal"
3. On the section "Add photos" upload a pdf file
4. Complete all the steps
5. On proposal's page, the pdf file is displayed under "

Expected behavior
Only images can be uploaded on the "Add image" section

Screenshots

Stacktrace
If applicable, add the error stacktrace to help explain your problem.

Extra data (please complete the following information):

• Device: [e.g. iPhone6, Desktop]
• Device OS: [e.g. iOS8.1, Windows 10]
• Browser: [e.g. Chrome, Firefox, Safari]
• Decidim Version: [e.g. 0.10]
• Decidim installation: [e.g. MetaDecidim]

Additional context
Add any other context about the problem here. For instance, add Metadecidim link.