In the Terminal, I've got the following output (runned with WINEDEBUG=warn+relay)
Marcs-MacBook-Pro:~ marcweber$ WINEDEBUG=warn+relay wine /Users/marcweber/.wine/drive_c/Program\ Files/Steam/Steam.exe -no-dwrite
fixme:heap:HeapSetInformation 0x0 1 0x0 0
fixme:process:SetProcessDEPPolicy (1): stub
fixme:heap:HeapSetInformation 0x0 1 0x0 0
Dynamic session lookup supported but failed: launchd did not provide a socket path, verify that org.freedesktop.dbus-session.plist is loaded!
fixme:heap:HeapSetInformation 0x0 1 0x0 0
ILocalize::AddFile() failed to load file "public/steambootstrapper_english.txt".
ILocalize::AddFile() failed to load file "public/steambootstrapper_german.txt".
[2013-12-04 20:47:14] Startup - updater built Dec 3 2013 18:35:30
fixme:advapi:EventRegister {47a9201e-73b0-42ce-9821-7e134361bc6f}, 0x3f005c90, 0x3f03ab30, 0x3f03ab28
fixme:advapi:EventRegister {58a9201e-73b0-42ce-9821-7e134361bc70}, 0x3f005c90, 0x3f03ab68, 0x3f03ab60
fixme:advapi:EventRegister {3fa9201e-73b0-43fe-9821-7e145359bc6f}, 0x3f005c90, 0x3f03aaf8, 0x3f03aaf0
fixme:advapi:EventRegister {1432afee-73b0-42ce-9821-7e134361b433}, 0x3f005c90, 0x3f03aba0, 0x3f03ab98
fixme:advapi:EventRegister {4372afee-73b0-42ce-9821-7e134361b519}, 0x3f005c90, 0x3f03abd8, 0x3f03abd0
fixme:heap:HeapSetInformation 0x0 1 0x0 0
[2013-12-04 20:47:14] Verifying installation...
[2013-12-04 20:47:14] Verification complete
fixme:advapi:EventRegister {47a9201e-73b0-42ce-9821-7e134361bc6f}, 0x3f005c90, 0x3f03ab30, 0x3f03ab28
fixme:advapi:EventRegister {58a9201e-73b0-42ce-9821-7e134361bc70}, 0x3f005c90, 0x3f03ab68, 0x3f03ab60
fixme:advapi:EventRegister {3fa9201e-73b0-43fe-9821-7e145359bc6f}, 0x3f005c90, 0x3f03aaf8, 0x3f03aaf0
fixme:advapi:EventRegister {1432afee-73b0-42ce-9821-7e134361b433}, 0x3f005c90, 0x3f03aba0, 0x3f03ab98
fixme:advapi:EventRegister {4372afee-73b0-42ce-9821-7e134361b519}, 0x3f005c90, 0x3f03abd8, 0x3f03abd0
fixme:process:SetProcessShutdownParameters (00000100, 00000000): partial stub.
err:winediag:SECUR32_initNTLMSP ntlm_auth was not found or is outdated. Make sure that ntlm_auth >= 3.0.25 is in your path. Usually, you can find it in the winbind package of your distribution.
fixme:ole:RemUnknown_QueryInterface No interface for iid {00000019-0000-0000-c000-000000000046}
[1204/204715:ERROR:resource_bundle.cc(417)] Failed to load C:\Program Files\Steam\bin\chrome.pak
Some features may not be available.
fixme:iphlpapi:NotifyAddrChange (Handle 0x5d7e188, overlapped 0x5b970c0): stub
fixme:winsock:WSALookupServiceBeginW (0x5d7e288 0x00000ff0 0x5d7e2d0) Stub!
[1204/204715:ERROR:network_change_notifier_win.cc(126)] WSALookupServiceBegin failed with: 8
fixme:winsock:WSAIoctl ignoring keepalive interval and timeout
fixme:ntdll:NtQuerySystemInformation info_class SYSTEM_HANDLE_INFORMATION
fixme:win:RegisterDeviceNotificationA (hwnd=0x50090, filter=0x107cf1ac,flags=0x00000000) returns a fake device notification handle!
Dec 4 20:47:15 Marcs-MacBook-Pro.local Steam.exe[3767] <Error>: ImageIO: PNG Extra compressed data
fixme:winsock:WSAIoctl ignoring keepalive interval and timeout
err:ole:CoGetClassObject class {77f10cf0-3db5-4966-b520-b7c54fd35ed6} not registered
err:ole:CoGetClassObject no class object {77f10cf0-3db5-4966-b520-b7c54fd35ed6} could be created for context 0x1
fixme:wbemprox:wbem_locator_ConnectServer unsupported flags
fixme:wbemprox:client_security_SetBlanket 0x487cf328, 0x20da50, 10, 0, (null), 3, 3, 0x0, 0x00000000
fixme:wbemprox:client_security_Release 0x487cf328
err:wbemprox:wql_error syntax error, unexpected TK_NOT
fixme:wbemprox:wbem_locator_ConnectServer unsupported flags
fixme:wbemprox:client_security_SetBlanket 0x487cf328, 0x20b5b8, 10, 0, (null), 3, 3, 0x0, 0x00000000
fixme:wbemprox:client_security_Release 0x487cf328
fixme:mountmgr:harddisk_ioctl The DISK_PARTITION_INFO and DISK_DETECTION_INFO structures will not be filled
fixme:wbemprox:enum_class_object_Next timeout not supported
fixme:wbemprox:enum_class_object_Next timeout not supported
fixme:wbemprox:enum_class_object_Next timeout not supported
fixme:wbemprox:enum_class_object_Next timeout not supported
err:ole:CoGetClassObject class {dff32fea-3331-48da-a272-ccfc238695be} not registered
err:ole:CoGetClassObject class {dff32fea-3331-48da-a272-ccfc238695be} not registered
err:ole:create_server class {dff32fea-3331-48da-a272-ccfc238695be} not registered
fixme:ole:CoGetClassObject CLSCTX_REMOTE_SERVER not supported
err:ole:CoGetClassObject no class object {dff32fea-3331-48da-a272-ccfc238695be} could be created for context 0x17
Marcs-MacBook-Pro:~ marcweber$ fixme:advapi:EventUnregister deadbeef: stub
fixme:advapi:EventUnregister deadbeef: stub
fixme:advapi:EventUnregister deadbeef: stub
fixme:advapi:EventUnregister deadbeef: stub
fixme:advapi:EventUnregister deadbeef: stub
Marcs-MacBook-Pro:~ marcweber$
By the way… what format is accepted as attachments? Neither .txt, nor .rtf are allowed

WTF