I can’t read/view attachment within online webmail/ Outlook Web App (OWA).

to help you secure your systems, WorldPosta by default blocks attachment with the following file name extensions:

vsmacros, .msh2xml, .msh1xml, .ps2xml, .ps1xml, .mshxml, .gadget, .mhtml, .psc2, .psc1, .msh2,.msh1, .aspx, .xml, .wsh, .wsf, .wsc, .vsw, .vst, .vss, .vbs, .vbe, .url, .tmp, .shs, .shb, .sct,.scr, .scf, .reg, .pst, .ps2, .ps1, .prg, .prf, .plg, .pif, .pcd, .ops, .mst, .msp, .msi, .msh,.msc, .mht, .mdz, .mdw, .mdt, .mde, .mdb, .mda, .maw, .mav, .mau, .mat, .mas, .mar, .maq, .mam,.mag, .maf, .mad, .lnk, .ksh, .jse, .its, .isp, .ins, .inf, .htc, .hta, .hlp, .fxp, .exe, .der,.csh, .crt, .cpl, .com, .cmd, .chm, .cer, .bat, .bas, .asx, .asp, .app, .adp, .ade, .ws, .vb, .js

Messages which contain the previous extensions are not allowed in OWA, Outlook desktop software and Outlook Apps.