.htaccess で MIME Type を設定して下さい。

application/activemessage
application/andrew-inset
application/applefile
application/atomicmail
application/dca-rft
application/dec-dx
application/dsptype		tsp
application/fractals		fif
application/mac-binhex40
application/mac-binhex40	hqx
application/mac-compactpro	cpt
application/macwriteii
application/msword		doc
application/news-message-id
application/news-transmission
application/octet-stream	bin exe scm dwg fli gl dms lha lzh
application/oda			oda
application/pdf			pdf
application/postscript		ai eps ps
application/powerpoint		ppt
application/remote-printing
application/rtf			rtf
application/slate
application/stuffit		sit
application/wita
application/wordperfect5.1
application/x-bcpio		bcpio
application/x-cpio		cpio
application/x-csh		csh
application/x-director		dcr dir dxr
application/x-dvi		dvi
application/x-envoy		evy
application/x-gtar		gtar
application/x-hdf		hdf
application/x-httpd-php		phtml
application/x-latex		latex
application/x-mif		mif
application/x-netcdf		nc cdf
application/x-sh		sh
application/x-shar		shar
application/x-sv4cpio		sv4cpio
application/x-sv4crc		sv4crc
application/x-tar		tar
application/x-tcl		tcl
application/x-tex		tex
application/x-texinfo		texinfo texi
application/x-troff		t tr roff
application/x-troff-man		man
application/x-troff-me		me
application/x-troff-ms		ms
application/x-ustar		ustar
application/x-wais-source	src
application/zip			zip
audio/basic			au snd
audio/midi			mid
audio/voxware			vox
audio/x-aiff			aif aiff aifc
audio/x-pn-realaudio		ram ra
audio/x-wav			wav
image/gif			gif
image/ief			ief
image/jpeg			jpeg jpg jpe
image/tiff			tiff tif
image/x-cmu-raster		ras
image/x-portable-anymap		pnm
image/x-portable-bitmap		pbm
image/x-portable-graymap	pgm
image/x-portable-pixmap		ppm
image/x-rgb			rgb
image/x-xbitmap			xbm
image/x-xpixmap			xpm
image/x-xwindowdump		xwd
message/external-body
message/news
message/partial
message/rfc822
multipart/alternative
multipart/appledouble
multipart/digest
multipart/mixed
multipart/parallel
text/html			html htm
text/plain			txt
text/richtext			rtx
text/tab-separated-values	tsv
text/x-setext			etx
text/x-sgml			sgml sgm
video/mpeg			mpeg mpg mpe
video/quicktime			qt mov
video/x-msvideo			avi
video/x-sgi-movie		movie
x-world/x-vrml			wrl vrml