This test fails because it expects a MIME type "Content-Type: image/gif"
whereas that provided by libmagic is "Content-Type: image/gif; charset=binary"
Presumably the test was written against a different version.
--- aegis-4.24/test/02/t0228a-matt.sh	2008-03-14 07:19:27.000000000 +0100
+++ aegis-4.24/test/02/t0228a-matt.sh	2014-01-27 09:43:44.000000000 +0100
@@ -147,7 +147,8 @@
 SCRIPT_NAME=/cgi-bin/aeget \
 PATH_INFO=icon/rss.gif \
 QUERY_STRING= \
-$bin/aeget > rss.served.gif 2>&1
+$bin/aeget \
+| sed -e 's%^Content-Type: image/gif; charset=binary%Content-Type: image/gif%' > rss.served.gif 2>&1
 if test $? -ne 0 ; then fail; fi
 
 # Compare the expected and the actual received file.
'h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
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<div class='path'>path: <a href='/guix/log/?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>root</a>/<a href='/guix/log/gnu?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>gnu</a>/<a href='/guix/log/gnu/packages?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>packages</a>/<a href='/guix/log/gnu/packages/gd.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>gd.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/gd.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/gd.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-14 16:36:58 -0800'>2021-11-14</span></td><td><a href='/guix/commit/gnu/packages/gd.scm?id=6cdf4e5bf230fdbe17e592c2ec74fb34dba70eb5'>gnu: gd: Fix long patch filename.</a><span class='msg-avail'>...<span class='msg-tooltip'>This fixes the generation of a tarball with "make dist":

  tar: guix-1.3.0.10365-4c4f9/gnu/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch:
  file name is too long (max 99); not dumped

* gnu/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch:
  Rename patch to ...
* gnu/packages/patches/gd-Revert-fix-303-gdlib.pc.patch: ... this.
* gnu/packages/gd.scm (gd)[source]: Update patch filename.
* gnu/local.mk (dist_patch_DATA): Update patch filename.
</span></span></td><td>Vagrant Cascadian</td></tr>
<tr><td><span title='2021-03-28 20:13:22 +0200'>2021-03-28</span></td><td><a href='/guix/commit/gnu/packages/gd.scm?id=b20b45c6ce67593db9d446d066e02310671ad8d7'>gnu: gd: Patch away recent pkg-config files change that breaks php build.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/gd.scm (gd): Apply patch.
</span></span></td><td>Léo Le Bouter</td></tr>
<tr><td><span title='2021-03-15 11:26:50 +0200'>2021-03-15</span></td><td><a href='/guix/commit/gnu/packages/gd.scm?id=0ca7e1909356967429309ecdf70a21f609413521'>gnu: gd: Update to 2.3.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gd.scm (gd): Update to 2.3.2.
[arguments]: Adjust comment on custom 'disable-failing-test phase.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-11-30 00:25:51 +0100'>2020-11-30</span></td><td><a href='/guix/commit/gnu/packages/gd.scm?id=4ac139631d255c20b4ef3a1d7adc2d05c3467091'>gnu: perl-gd: Update to 2.73.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/gd.scm (perl-gd): Update to 2.73.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>