Improve bp_core_check_avatar_type() and introduce a function to get the allowed avatar types

[imath]

During WC London, DJPaul recommended:

• to improve the i18n of #6278 for the error message when a file is not a supported image file type.
• bp_core_check_avatar_type (and related functions) should be committed separately and have its own ticket.

I've attached a unit test 'current_check_avatar_type.patch' which is only testing the current bp_core_check_avatar_type(). This unit test requires 2 files i'm not attaching

• tests/phpunit/assets/files/buddypress_logo.pdf
• tests/phpunit/assets/files/disc.png

You'll see that the current function is failing the last assertion.

In 01.patch, i'm suggesting 2 new functions :

• bp_core_get_allowed_avatar_types() which include a filter to restrict image types
• bp_core_get_allowed_avatar_mimes() to get the corresponding mime types

I think bp_core_check_avatar_type() should use wp_check_filetype_and_ext() to be absolutely sure an image was uploaded.

[imath]

6336.02.patch is improving 01.patch

[imath]

Adds a unit test for bp_core_get_allowed_avatar_mimes()

[imath]

Use mystery-man.jpg and humans.txt instead of adding files in tests assets

[imath]

In 9760:

Improve bp_core_get_allowed_avatar_types(). Make sure to get the file type if the upload was not done using html5

The way we used to check the image file type is not working if the browser is Internet Explorer < 10. In this particular case, the Plupload runtime is falling back to flash and the file type is application/octet-stream. We are now using wp_check_filetype_and_ext() to fix this issue.

You can use the filter bp_core_get_allowed_avatar_types if you wish to *restrict* the avatar image types. Our supported types are: jpg, jpeg, png and gif.

Props DJPaul.

Fixes #6336
See #6290
See #6278