Reproduction steps:
1. Open the attached project;
2. Click on the "MT_DLG_RoundFrame_DC" in the Project folder;
3. Open the Inspector window;

Actual result: image is imported like RGB image, but the image has alpha channel (actualLook.jpg).
Expected result: an image should be imported like RGBA image (expectedLook.jpg).

Note: the issue happens because imported image is converted into DXT1 format which does not have alpha channel. In the 5.3 and 5.4 versions, the same image is converted into DXT5 format which has alpha channel. Also, in some versions (5.4.5p1, 5.5.2f1, 2017.1.0a5) an image is imported correctly, but in the Preview window it still looks like RGB image (stillWrongLook.jpg).

Reproduced with: 5.5.2p4, 5.6.0f2.
Not reproducible with: 5.3.7p4, 5.4.5p1 (in the Preview window, the image still looks like RGB), 5.5.2f1 (in the Preview window, the image still looks like RGB), 2017.1.0a5 (in the Preview window, the image still looks like RGB).
Regression introduced in 5.5.2 patch