Prerequisite:
You'll need to install tool that opens .hdr files (photoshop, hdr viewer, etc).

Steps to repro:

1. Open attached project;
2. Select 'Arches_E_PineTree_3k' file in Assets folder an open it in any external .hdr image viewer tool;
3. Observe texture preview in Inspector and compare it to the external viewing tool.

Actual result:
HDR texture preview is significantly darker than real image in Linear color. See attached actual.png.

Expected result:
HDR texture preview rendering should be same with external tool's one. You can observe expected outcome by switching to Gamma color space.
See attached expected.png image.

Notes:
- Reproducible in 5.6.0a3, 5.6.0a2, 5.5.0f3, 5.4.3f1;
- Reproducible on both OSX and Windows;
- Doesn't repro on rendered texture assigned to material's property.