Solution : https://service.sap.com/sap/support/notes/550669 (SAP Service marketplace login required)
Key words : 
sap->bw->bex            sap->bw->mime            sap->bw->doc->browser            sap->bw->doc->hier            sap->bw->doc->mast            sap->bw->doc->meta            sap->bw->doc->metadata            sap->bw->doc->tmpl            sap->bw->doc->tran            caution, bw-relevant icf services, goto -> http server cache -> invalidate -> global, symptom bw web applications, bw web applications, bw support packages, web browser, sapbwnews bw 3, firewalls prevent content encoding, http server cache
Related Notes : 
       
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| 580776 | CO_COMPRESS_IN_ALL_CASES constant has no influence | 
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| 577275 | Explicitly suppressing HTTP compression (gzip) | 
| 568322 | Exporting from BW Web reports to Excel (download) | 
| 567746 | Composite note BW 3.x performance: Query & Web applications | 
| 561792 | Client-sided caching of image/gif files | 
| 560980 | Compressed transfer of MIMEs with Netscape Navigator | 
| 556854 | Compressed transfer and Browser caching of MIME files | 
| 545741 | Additional information on upgrading to BW 3.1 Content | 
| 539820 | SAPBWNews BW 3.1 Content Support Package 03 | 
| 523243 | SAPBWNews BW 3.0B Support Package 09 | 
| 510835 | Additional information on upgrading to BW 3.0B |