Improve `bp_attachments_get_allowed_types()`

[imath]

Just found this function was wrongly getting video extensions for the audio ones.

[imath]

In 13132:

Improve the bp_attachments_get_allowed_types() function

• Make sure requesting for the 'audio' type returns a list of audio file extensions.
• Use the wp_get_ext_types() function so that passing the 'image', 'document', 'spreadsheet', 'interactive', 'text', 'archive', 'code' types to the function will return the requested type extensions.

Fixes #8595

[imath]

In 13134:

Improve 2 Core Attachments functions

• stop using a switch() statement in favor of some if/else ones to avoid a possible notice error when getting all allowed BP Attachements.
• stop using wp_match_mime_types() inside the bp_attachments_get_allowed_mimes() function. Using wp_match_mime_types() was generating unconsistent results when trying to get all BP Attachments allowed mime types.

Fixes #8595 (really)